Fix result-handling logic in wrapped plugin classes

This fixes the handling of errors in the measureN methods, which were broken in
DM-984 (we need tests for those, but we don't have any plugins that implement
measureN yet).  It also avoids the working-but-bad-form use of instance attributes
to pass the result object between methods.
Cleanup GaussianFlux test, use more test utility code
Remove relic code from GaussianFlux

This looks like a combination of copy/paste relics from PsfFlux, as well
as some config options that should have been removed in DM-1015, when we
stopped having a use for them.
Bail out early if one of a plugin's dependencies failed

This currently only applies to shapes, because we fall back to using the Peak
when a centroid measurement fails.
Don't pass shapeFlag into GaussianFlux; it's checked before we get that far
Add aliases to flags of dependencies in dependent algorithms.
Add test for shape dependency error handling in GaussianFlux
Merge branch 'u/jbosch/DM-1332'
Merge branch 'master' into u/pgee/DM-1217