Only the "ycm:ident" value of the ft= argument in the YcmCompleter
command is working. This value forces the use of the identifier
completer which serve no purpose because this completer doesn't define
commands. Ignore completely the ft= argument and move the logic to the
Python layer.
Evaluating the values of g:ycm_extra_conf_vim_data may raise a Python exception
(e.g. one of the values is not defined). Since that option is parsed each time
a request is sent, such exception makes the editor almost unusable as each key
press is printing a Python traceback to the user. Catch and log the exception.
