Minor string changes in libgdata and GLib

[Philip Withnall <philip tecnocode co uk>]

Hi all,

A couple of small string changes in libgdata and GLib before the string
freeze on 2019-08-19. Both changes are to strings which the user will
not commonly see (a string in an error message in libgdata, and a
string in a developer tool error message in GLib). Both changes are to
convert a British English spelling into a US English one, so shouldn’t
affect any translations except en_* ones.

https://gitlab.gnome.org/GNOME/libgdata/merge_requests/15
https://gitlab.gnome.org/GNOME/glib/merge_requests/1038

Philip