Provide a better error message when language is not recognised

lib/config-utils.js

@@ -23,6 +23,13 @@ const QUERIES_PROPERTY = 'queries';
 const QUERIES_USES_PROPERTY = 'uses';
 const PATHS_IGNORE_PROPERTY = 'paths-ignore';
 const PATHS_PROPERTY = 'paths';
+// All the languages supported by CodeQL
+const ALL_LANGUAGES = ['csharp', 'cpp', 'go', 'java', 'javascript', 'python'];
+// Some alternate names for languages
+const LANGUAGE_ALIASES = {
+    'c': 'cpp',
+    'typescript': 'javascript',
+};
 /**
  * A list of queries from https://github.com/github/codeql that
  * we don't want to run. Disabling them here is a quicker alternative to
@@ -350,7 +357,28 @@ async function getLanguages() {
         throw new Error("Did not detect any languages to analyze. " +
             "Please update input in workflow or check that GitHub detects the correct languages in your repository.");
     }
-    return languages;
+    // Make sure they are supported
+    const checkedLanguages = [];
+    const unknownLanguages = [];
+    for (let language of languages) {
+        // Normalise to lower case
+        language = language.toLowerCase();
+        // Resolve any known aliases
+        if (language in LANGUAGE_ALIASES) {
+            language = LANGUAGE_ALIASES[language];
+        }
+        const checkedLanguage = ALL_LANGUAGES.find(l => l === language);
+        if (checkedLanguage === undefined) {
+            unknownLanguages.push(language);
+        }
+        else if (checkedLanguages.indexOf(checkedLanguage) === -1) {
+            checkedLanguages.push(checkedLanguage);
+        }
+    }
+    if (unknownLanguages.length > 0) {
+        throw new Error("Did not recognise the following languages: " + unknownLanguages.join(', '));
+    }
+    return checkedLanguages;
 }
 /**
  * Get the default config for when the user has not supplied one.