gbookmarkfile: Cleaner error handling code to pacify static analysis

A static analyzer flagged the g_file_get_contents() call as not
checking its return value.  While the code here is actually correct,
it's verbose at best.

I think the "goto out + cleanup" code style is substantially cleaner,
less error prone, and easier to read.  It also will pacify the static
analyzer.

https://bugzilla.gnome.org/show_bug.cgi?id=731584