feat: report parse failure reasons

For [](#1051)

I opted for the more basic logging style - I don't know enough about
the diagnostics objects to be sure that they'll always contain the same
fields, so erred on the side of caution rather than trying to access
fields within fields that aren't guaranteed to exist (as well as
including all available information, regardless of what is provided).

lib/generator.js

@@ -353,6 +353,9 @@ class Generator {
       if (!document) {
         const err = new Error('Input is not a correct AsyncAPI document so it cannot be processed.');
         err.diagnostics = diagnostics;
+        for (const diag of diagnostics) {
+          console.error('Diagnotic err:', diag);
+        }
         throw err;
       } else {
         this.asyncapi = document;