Add SkipError for indicating a skipped activity

SkipError can now be raised within the context of a timed_activity() to
trigger a SKIPPED message instead of a FAILURE message.

This changes the previous usage of SKIPPED within messages to use the
INFO MessageType (used within cascache.py).

Element._pull() now performs its actions from within a timed_activity()
to match Element._push() and allow for SkipError to be raised.

"Downloaded artifact <cache key>" has been reworded to "Pulled artifact
<cache key>" for consistency with Element._push().