Rename importers.html to default_importers.html to clarify its role
Rename generate_trace_viewer to be vulcanize_trace_viewer so its role is clearer
Create trace_viewer/trace_viewer.html as toplevel viewer component
Fix handling of errors during module loading

The semantics of a.then(b,c).then(d,e) are that if a rejects, but c does not
throw, then d will run. I had originally assumed that if a rejects, then c and
e must both run all the time. This then led to missing ui in the test runner
when loading failed.
Introduce ViewBase and move view registration handling to there.
Move importer registration to Importer base class.