Async TTM cleanup: Moved ret error checking into a function.

* Moved ret and ret.tokens error checking into a separate function
  and removed piecemeal checking from couple places.  This also
  reported a callback site in TemplateHandler:fetchArg that was
  returning a string instead of an array of tokens.  Now fixed.

* Added additional inline comments to transformTokens loop in
  AsyncTTM and fairly minor cleanup.

* No change in parser test results.

* Verified errorless RTing of en:Barack Obama.

Change-Id: I2568452e641e9ebae0eb772276a5fe11cccbde51