Only warn about unknown tags in instrumentation.xml

Only warn about unknown tags in instrumentation.xml

Richard J. Senior:
Without this commit, unknown top-level tags in instrumentation.xml are treated
as errors and the rest of the file is not processed.

Users running Flightgear versions prior to 3.2 have experienced problems with
the comm-radio tags after downloading updated versions of aircraft from FGADDON.
Instrumentation listed after the unrecognized comm-radio tags is not processed
for these users and is inactive in the cockpit.

This commit changes the instrumentation build method so that unrecognized tags
are treated as warnings. This won't help users running older versions but
protects against the same problems occurring if new tags are added to
instrumentation in the future.