1914415 - Show a better error message if `uplift` creation fails, e.g. because the changeset is already present in the destination

Status: NEW

(Conduit :: moz-phab, enhancement, P3)

Description

[Jonathan Kew [:jfkthame]]

moz-phab version (from moz-phab --version): MozPhab 1.5.1 (Python 3.11.9, Darwin)
Python version (from python3 --version): Python 3.12.3
OS: macOS 14.1

I was attempting to do moz-phab uplift -s <changeset>, and getting a large block of error text beginning

ServerError: server exited with status 1: b'** Unknown exception encountered with possibly-broken third-party extension "clang-format" (version N/A)\n** which supports versions 5.3 of Mercurial.\n** Please disable "clang-format" and try your action again.\n

and lots more, all as one long string.

I spent entirely too much time trying to figure out what was broken about my setup, before eventually realizing that the changeset I was trying to uplift is already present in beta (and so any attempt to transplant it was of course doomed to fail). This was not at all clear from the moz-phab failure, though; it looked for all the world like the tool was broken/misconfigured.

Ideally, if an attempt to transplant/rebase the changeset(s) fails, that failure would be clearly reported to the user, instead of the spurious "broken mercurial" messages that pointed me in entirely the wrong direction.