phlurl_request: wrap httplib exceptions, add info

Make exceptions that occur during phlurl_request.get*() easier to
understand by adding the context of the current operation to them, e.g.
'Was trying to GET from http://www.server.test/'

Make exceptions emitted from phlurl_request easier to catch by wrapping
them in a new phlurl_request.Error type.

Test Plan:
$ ./precommit.sh

$ ./testbed/arcyd/test_shell.sh
  $ rm -rf ../origin
  observe exception displayed on Arcyd dashboard
Arcyd: remove enhanced traceback reporting

Don't send the 'enhanced traceback' report when handling an exception,
they don't seem to have ever helped and seem to occasionally crash
during reporting.

Remove phlsys_traceback component.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh
$ git grep phlsys_traceback
- observe empty result