app: gimp_procedure_execute(): don't use a NULL plug-ins error message

return_vals[1] being present and being a string doesn't neccessarily
mean it comes from the plug-in, it might just as well be its first
return value, which gets generated and NULL-initialized by the plug-in
execution code even if the plug-in didn't actually return anything.
libgimpcolor: add gimp_lcms_profile_get_summary()

Which returns an "executive summary" of the profile, much like lcms1's
"profile info" API. Refactor common code out of the other string
getters.
modules: use gimp_lcms_profile_get_summary() in the lcms display filter
app: be more robust about possible profile info return values
plug-ins: use gimp_lcms_profile_get_summary() in the lcms plug-in
app: move all GimpProfileChooserDialog to a private struct

and make the preview widget look the same as the profile tab
in the image properties dialg.