Handle MarkerNotFound better in Flavor API

When the specified marker is not found, we should catch the
MarkerNotFound exception and respond with a HTTP 400 Bad
Request with sufficient information back to the user. This
will avoid spilling unwanted tracebacks in the logs as well.

Change-Id: I90a658384f8814096271a193a0f3abd028062b5d
Partial-Bug: 1238519
Merge "Handle MarkerNotFound better in Flavor API"