Improved error handling for most calculations

app/imports/parser/parser.js

@@ -28,10 +28,21 @@ export function parse(string){
   if (results.length === 1){
     return results[0];
   } else if (results.length === 0){
-    // Valid parsing up until now, but need more. Unexpected end of input.
-    return null;
+    // Valid parsing up until now, but need more
+    throw new EndOfInputError('Unexpected end of input');
   } else {
     console.warn('Grammar is ambiguous!', {string, results});
     return results[0];
   }
 }
+
+export function prettifyParseError(e){
+  if (e.message) e = e.message
+  return e.toString().split('.')[0];
+}
+
+class EndOfInputError extends Error {
+  constructor(message = '', ...args) {
+    super(message, ...args);
+  }
+}