Reset migrating task state for MigrationError exceptions

Add MigratingError to the list of exceptions for which instance task
state is reset to ACTIVE to avoid it being stuck in MIGRATING state.

This should be safe to add as all occurances where MigrationError is
being raised happens during pre-migrating checks before the actual
migration takes place.

In addition, also make sure that expected exceptions are passed over
RPC silently.

Resolves bug 1171526.

Change-Id: I00fa0962e555f895dd98861684ec7767fed3b37b