RFR: 1000: mlbridge stuck failing on closed PR targeting a branch that no longer exists

[Erik Joelsson]

This patch fixes two related issues in mlbridge where the bot would fail with exception (and subsequently retry indefinitely while spamming admins with alarms) if the target branch or the integrated change of a PR no longer exists. My conclusion on how to fix this is to simply log a warning if either of these cases are detected and stop processing more.

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Commit messages:
 - SKARA-1000

Changes: https://git.openjdk.java.net/skara/pull/1161/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1161&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1000
  Stats: 81 lines in 3 files changed: 78 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1161.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1161/head:pull/1161

PR: https://git.openjdk.java.net/skara/pull/1161