[WFS] Produce exception if gml_featureid item not valid

[dmorissette]

If a WFS layer contains the wfs/gml_featureid metadata and the item in the
metadata is not found in the database, then we should issue an exception.

At the moment, if the item is not found we just silently ignore the featureid,
resulting in invalid WFS responses (i.e. without fid) that can be hard to debug
with some clients.  (The WFS spec requires the fid attribute in GetFeature
responses.)

If I hear no objections then I'll make this change in 4.9

[dmorissette]

Doing this now.

[dmorissette]

Done in 4.9/4.10.

Producing an exception would have been a bit tricky because we test for the
featureid item only in the middle of a GetFeature output and at that time it's
too late to produce an exception, so what I did instead is produce a warning in
the GetFeature output. Not perfect but better than nothng.