Without on_error() handler curl.get() will raise an error which then
will interrupt execution of the plenary job. This interruption will
result in timeout handling code triggering, introducing unneeded delay
and ugly stack traces.
Fix the problem by defining on_error() and callback() handlers and
explicitly wait for job completion with pcall() and Job:wait(). This
allows proper error handling and control over error messages.
Because plenary's curl implementation mangles stderr data, use a local
table to map select error codes to descriptive text messages.
Also do not emit hard error when endpoint is not configured, use
Utils.error().
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