Action: Better error when a platform is not available for a GCC version.

README.md

@@ -62,6 +62,15 @@ section.
 - `4.8-2014-q3`    `4.8-2014-q2`   `4.8-2014-q1`   `4.8-2013-q4`
 - `4.7-2014-q2`    `4.7-2013-q3`   `4.7-2013-q2`   `4.7-2013-q1`
 
+Older GCC version might not have releases for all operating system
+architectures, specifically `arm64`.
+This table shows the first release compatible with each OS architecture.
+
+|         | x86_64      | arm64                  |
+|---------|-------------|------------------------|
+| Linux   | All version | From version 9-2019-q4 |
+| macOS   | All version | From version 12.2.Rel1 |
+| Windows | All version | Not supported          |
 
 ## Advanced options
 

__tests__/main.test.ts

@@ -93,12 +93,12 @@ test('test fetching urls for invalid platforms', () => {
   // macOS arm64 starts at 12.2.Rel1
   expect(() => {
     gcc.distributionUrl('11.3.Rel1', 'darwin', 'arm64');
-  }).toThrow('invalid platform mac_arm64');
+  }).toThrow('invalid platform mac_arm64 for GCC version 11.3.Rel1');
 
   // Linux aarch64 starts at 9-2019-q4
   expect(() => {
     gcc.distributionUrl('8-2019-q3', 'linux', 'arm64');
-  }).toThrow('invalid platform linux_aarch64');
+  }).toThrow('invalid platform linux_aarch64 for GCC version 8-2019-q3');
 
   // Invalid release id
   expect(() => {

dist/index.js

@@ -592,7 +592,8 @@ function distributionUrl(version, platform, arch) {
         throw new Error(`invalid GCC version ${version}. Available: ${availableVersions()}`);
     }
     if (!versions[version].hasOwnProperty(osName)) {
-        throw new Error(`invalid platform ${osName}.`);
+        throw new Error(`invalid platform ${osName} for GCC version ${version}.\n` +
+            'The action README has the list of available versions and platforms.');
     }
     return versions[version][osName];
 }

src/gcc.ts

@@ -702,7 +702,10 @@ export function distributionUrl(version: string, platform: string, arch: string)
     throw new Error(`invalid GCC version ${version}. Available: ${availableVersions()}`);
   }
   if (!versions[version].hasOwnProperty(osName)) {
-    throw new Error(`invalid platform ${osName}.`);
+    throw new Error(
+      `invalid platform ${osName} for GCC version ${version}.\n` +
+        'The action README has the list of available versions and platforms.'
+    );
   }
   return versions[version][osName];
 }