flyctl reports "An unknown error occured." for some release_command values

I made a typo in fly.toml release_command.

# troubleshooting temporary value
release_command = "sh -c '/ && something'"
# I intended to change it back to
release_command = "something"
# but I accidentally left in a single quote:
release_command = "something'"

This causes a weird & unhelpful error message from flyctl deploy:

==> Creating release
Error An unknown error occured.

Thanks for the report. This is indeed an issue with the mismatched quote.

The reason you see a generic error: for security reasons, we generally try to display detailed errors only when we’ve vetted the error or generated it ourselves. As opposed to sending any error down that might contain sensitive information about our systems. This is a good example of an error we could catch and make explicit!