shell: fatal error log messages are lost

[grondo]

Problem: Just after the shell connects to the broker and a flux_t handle is available, the evlog based shell logger disables log output to stderr completely. However, at this time there might not yet be another destination for these messages, and so they are lost. For example, when the job shell gets an error parsing jobspec, the user will only see:

flux-job: task(s) exited with exit code 1
flux-job: No job output found

If there is no other destination for a fatal error log, the shell should somehow detect that and write the message to stderr so that the job-exec module can add it to the eventlog.