write_common: Make check_writer_arg() exit on syntax errors.

write_common: Make check_writer_arg() exit on syntax errors.

This function parses the argument of the --writer option to extract
the name of the writer, which is the first word of the argument. If
this word is not the name of a supported writer, the function
returns NULL to indicate that the given writer does not exist.
Otherwise check_writer_arg() calls the ->parse_config_or_die method
of this writer which parses the remaining part of the argument and
exits on errors.

It is more consistent and simpler to let this function exit also
in case the writer was not found. This simplifies the callers a bit
since they do not have to check the return value any more. It also
allows to kill E_WRITE_COMMON_SYNTAX.