Fixing race condition on server crash

Crash handling code tried to read stderr log file, but the file can be deleted
by the server befor we get to look at it. In such a case, we post a different
error message (without the log output).

Point is we do our best to get the error output if the user doesn't have
g:ycm_server_keep_logfiles set. If we can't get the logfile, oh well. We tell
the user to set that flag if they care.

Fixes #974.