Importing invalid subtrace eventdata shouldn't throw error.

When importing zip, gzip traces, the archive might contain eventData
which is not understandable by any of the existing importers.
Trace-viewer doesn't know how to handle that eventData and it throws
no importer error and stops import.

Trace-viewer should throw error only when it doesn't find even a single
valid importer. If we find any valid importer for any part, we should
contiune and import the valid part.

BUG=613