Improve metric handling for HTTPErrors
Depend on preq 0.2.11

Tests were failing with 0.2.10, as that wasn't returning instances of the
global Promise object.
Convert 'deleted' attributes into a tag set

We need to express various restrictions of a revision, including 'deleted' and
'suppressed'. A simple way to do so is via a set of string tags. This patch
removes the old boolean 'deleted', and replaces it with a set of strings.
Merge pull request #80 from gwicke/error-handling

Minor error handling, preq & revision deletion stuff