RFR: 1984: SKARA bot couldn't handle jep command if the jep doesn't have JEP Number

[Zhao Song]

A user issued command /jep JDK-8310626 in https://github.com/openjdk/jdk/pull/15103 and skara bot kept throwing exceptions because the JEP doesn't have a JEP number. 

@kevinrushforth suggested that it makes more sense to make skara bot be able to associate a PR with a JEP before it is a Candidate.

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

Commit messages:
 - SKARA-1984

Changes: https://git.openjdk.org/skara/pull/1541/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1541&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1984
  Stats: 63 lines in 3 files changed: 57 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/skara/pull/1541.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1541/head:pull/1541

PR: https://git.openjdk.org/skara/pull/1541