Differentiate between error types when failing to load an exercise.

Summary:
We used to only blame the user's internet.  But it seems sometimes it is
our fault too.  So now we differentitate between the different errors,
to be less confusing to users whose internet is not to blame, and to
help devs debug any particular issue based upon what our users report.

This is to fix: https://app.asana.com/0/27216215224639/71186369010999

Test Plan:
git grep warnTimeout
see that we weren't using this anywhere else.

on dev khan exercise files if they exist get served directly.  So to test we
can change the name so that it uses the exercises/handler_raw.py.
```
diff --git a/khan-exercise.js b/khan-exercise.js
index 8a9bcfd..eb048c3 100644
--- a/khan-exercise.js
+++ b/khan-exercise.js
@@ -2048,7 +2048,7 @@ function loadExercise(exerciseId, fileName) {

     debugLog("loadExercise start " + fileName);
     // Packing occurs on the server but at the same "exercises/" URL
-    $.get(urlBase + "exercises/" + fileName).done(function(data) {
+    $.ajax({url:urlBase + "exercises/Z" + fileName, timeout:1000}).done(function(data) {
         debugLog("loadExercise got " + fileName);

         // Get rid of any external scripts in data before we shove data
```
http://www.khanacademy.dev/math/arithmetic/addition-subtraction/basic_addition/e/addition_1
See 404 message.

Then in webapp:
```
diff --git a/exercises/handler_raw.py b/exercises/handler_raw.py
index 5aa90bd..03a1c8f 100644
--- a/exercises/handler_raw.py
+++ b/exercises/handler_raw.py
@@ -17,6 +17,8 @@ class RawExercise(request_handler.RequestHandler):
         would also cause problems for the a/b tests in
         exercises.file_experiments.
         """
+        import time
+        time.sleep(5000)
         path = self.request.path
         filename = urllib.unquote(path.split('/', 3)[3])
```
http://www.khanacademy.dev/math/arithmetic/addition-subtraction/basic_addition/e/addition_1
See timeout message.

Then in webapp:
```
diff --git a/exercises/handler_raw.py b/exercises/handler_raw.py
index 5aa90bd..9e1728a 100644
--- a/exercises/handler_raw.py
+++ b/exercises/handler_raw.py
@@ -17,6 +17,7 @@ class RawExercise(request_handler.RequestHandler):
         would also cause problems for the a/b tests in
         exercises.file_experiments.
         """
+        1/0
         path = self.request.path
         filename = urllib.unquote(path.split('/', 3)[3])
```

See the generic error message.

Then in javascript console do:
MathJax.Ajax.Require("some.js")

See the MathJax error message mentioning some.js.

Reviewers: emily, lauren

Subscribers: kevindangoor

Differential Revision: https://phabricator.khanacademy.org/D24180

build/kathjax-config.js

@@ -119,7 +119,7 @@ MathJax.Ajax.timeout = 60 * 1000;
 MathJax.Ajax.loadError = (function( oldLoadError ) {
     return function( file ) {
         if (window.Khan) {
-          Khan.warnTimeout();
+          Khan.warnMathJaxError(file);
         }
         // Otherwise will receive unresponsive script error when finally finish loading
         MathJax.Ajax.loadComplete = function( file ) { };

khan-exercise.js

@@ -227,16 +227,21 @@ var Khan = {
         "constructions": ["kmatrix"]
     },
 
-    warnTimeout: function() {
-        $(Exercises).trigger("warning", [i18n._("Your internet might be too " +
-                "slow to see an exercise. Refresh the page or " +
+    warn: function(msg) {
+        $(Exercises).trigger("warning", [msg + i18n._(" Refresh the page or " +
                 "<a href='' id='warn-report'>report a problem</a>."),
                 false]);
         // TODO(alpert): This event binding is kind of gross
         $("#warn-report").click(function(e) {
             e.preventDefault();
-            $("#report").click();
+            $(".report-issue-link").click();
         });
+
+    },
+
+    warnMathJaxError: function(file) {
+        Khan.warn(i18n._("Sorry!  There seems to be an issue with our math " +
+                    "renderer loading the file: %(file)s.", {file: file}));
     },
 
     warnFont: function() {
@@ -2116,7 +2121,15 @@ function loadExercise(exerciseId, fileName) {
         });
     }).fail(function(xhr, status) {
         debugLog("loadExercise err " + xhr.status + " " + fileName);
-        Khan.warnTimeout();
+        if (status === "timeout") {
+            Khan.warn(i18n._("Your internet might be too slow to see an " +
+                             "exercise."));
+        } else if (xhr.status === 404) {
+            Khan.warn(i18n._("Oops!  We can't seem to find this exercise."));
+        } else {
+            Khan.warn(i18n._("Oops!  There was a problem loading this " +
+                             "exercise."));
+        }
     });
 
     return promise;