Fix detecting of broken "gtk" dependencies

Carles Pina i Estanyrequested to merge
carlespina/reportbug:fix-detecting-gtk into master

Not having the package "gir1.2-gtksource-4" the code was raising ValueError which was not captured via the "except ImportError".

The users were seeing:

$ reportbug --ui gtk Cannot use UI gtk because of "Namespace GtkSource not available", defaulting to "text" Please enter the name of the package in which you have found a problem, or type 'other' to report a more general problem. If you don't know what package the bug is in, please contact debian-user@lists.debian.org for assistance.

$ reportbug --ui gtk Cannot use UI gtk because of "Please install the reportbug-gtk package to use this interface.", defaulting to "text" Please enter the name of the package in which you have found a problem, or type 'other' to report a more general problem. If you don't know what package the bug is in, please contact debian-user@lists.debian.org for assistance.

which is consisting with that the code intended to do.

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