R1:4068ee2a752d

Make permanent worker failures more user-friendly

Summary:
Ref T11309. In that task, a user misunderstood two parts of this error:

- They took "exception" to mean "unexpected failure", when it was intended to mean "rare circumstance".
- They intereted the internal ID number of a commit to mean that Phabricator was malfunctioning.

Make the language of this condition more direct, explaining what the situation means in greater detail.

Additionally, we would previously re-throw this exception, which…