More accurate error messages when setting target release after mupdate - #11018

control: job assigned to worker 01KZ9P5EP0YS3SJVA73SXT0XCS [factory aws, i-0c5db66b9067d27a6] (queued for 50 s)
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task: starting task 0: "setup"
task: process exited: duration 4074 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 17214 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 16 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 11313 ms, exit code 0
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task: starting task 4: "build"
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|  Documenting omicron-uuid-kinds v0.1.0 (/work/oxidecomputer/omicron/uuid-kinds)
|  Documenting dev-tools-common v0.1.0 (/work/oxidecomputer/omicron/dev-tools/common)
|     Finished `dev` profile [unoptimized + debuginfo] target(s) in 7m 24s
| warning: the following packages contain code that will be rejected by a future version of  [...]
| note: to see what the problems were, use the option `--future-incompat-report`, or run `ca [...]
|    Generated /work/oxidecomputer/omicron/target/doc/api_identity/index.html and 203 other  [...]
| 
| real     7:24.674192038
| user    20:05.785676227
| sys      4:19.998755273
| trap        0.301701381
| tflt        0.026922184
| dflt        0.040010425
| kflt        0.000062760
| lock  7:04:23.775916316
| slp   2:29:21.081129028
| lat        34.369021758
| stop     2:09.094315155
task: process exited: duration 834037 ms, exit code 0
|W| found 0 output files