Refactor error handling code to fix an error caused by signup failure

This pull requests includes a fix to handle an error handling code which is used when signup fails.  The fix is to re-throw an error when a user already exists and let fail clause handle this condition, rather than handling it separately.

Also, removed error clause in signup/signin methods in auth services to return error back to user controller so that it can handle error directly.

cc: @jjmerino, @kmeurer, @jamesongamble, @suprbh