If `jobstart()` fails, then the subsequent `rpcrequest()` will throw due
to an invalid channel id. This causes `job.stderr` not to exist, so we
throw another exception when trying to dump the job's stderr.
Error detected while processing function remote#define#AutocmdBootstrap[1]..remote#host#Require[10]..provider#pythonx#Require:
line 22:
E716: Key not present in Dictionary: stderr
This obfuscates the actual problem.
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