GVariant: add way to print context of parse errors

This was a feature intended from the very beginning that somehow never
got written.  It's a way to replace these sort of error messages out of
the GVariant parser:

  1-2,10-15:unable to find a common type

with something in the style of the Vala compiler:

  unable to find a common type:
    [1, 2, 3, 'str']
     ^        ^^^^^

https://bugzilla.gnome.org/show_bug.cgi?id=715028