[mathematica mode] Fix stuck-stream error on unrecognized syntax

Author: jvanderloo-atlassian

mode/mathematica/mathematica.js

@@ -126,6 +126,7 @@ CodeMirror.defineMode('mathematica', function(_config, _parserConfig) {
     }
 
     // everything else is an error
+    stream.next(); // advance the stream.
     return 'error';
   }