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

control: job assigned to worker 01KZ9P5H3Z5WGGKMMYY8P7GZY4 [factory aws, i-009aff4a3eaf435a0] (queued for 54 s)
...
task: starting task 0: "setup"
task: process exited: duration 4134 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 17095 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 20 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 10890 ms, exit code 0
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task: starting task 4: "build"
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| a out/probe.tar.gz 3037K
| a out/propolis-server.tar.gz 34642K
| a out/pumpkind-gz.tar 26647K
| a out/switch-softnpu.tar.gz 503249K
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| real     1:30.085365001
| user     1:27.057096352
| sys         5.043036267
| trap        0.006909992
| tflt        0.000000000
| dflt        0.000000000
| kflt        0.000000000
| lock        0.000000000
| slp      1:26.936513207
| lat         1.122851381
| stop        0.000120610
task: process exited: duration 1224764 ms, exit code 0
|W| found 1 output files
|W| uploading: /work/package.tar.gz (2779440005 bytes)
|W| uploaded: /work/package.tar.gz