RFR: Allow a remote pre-integration branch to be missing

[Robin Westberg]

If a pre-integration branch doesn't exist remotely, there's no need to try to delete it.

-------------

Commit messages:
 - Allow a remote pre-integration branch to be missing

Changes: https://git.openjdk.java.net/skara/pull/892/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=892&range=00
  Stats: 51 lines in 2 files changed: 49 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/892.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/892/head:pull/892

PR: https://git.openjdk.java.net/skara/pull/892