launcher: refactor handling errors in meta_launcher_new

Fill in missing error checks and use g_error, since any
error in this phase would lead to crash later anyway.
This way we'll at least know what went wrong.

Although it was not the patch's original intention, it fixes
https://bugzilla.gnome.org/show_bug.cgi?id=754520

https://bugzilla.gnome.org/show_bug.cgi?id=753434