* Remove separate "internal error" from exec
* Fix unknown command error regex
* Add message about command not found regex
* Silence errors while reading files in exec
The stdout and stderr files may never be opened or written to in certain
circumstances. In particular, if the timeout is short enough, the child
node process does not have enough time to start, and the child script
does not execute, so the files are not written to. So, catch errors form
trying to read the files, and ignore them.
* Do not silence errors due to short timeouts
* Simplify test regex for missing command
* Default error code to 1 if not set
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