Show a better error message when the config file can't be found.

Review Request #10057 — Created July 3, 2018 and submitted — Latest diff uploaded

Information

rb-gateway
master
8437235...

Reviewers

If the config file doesn't exist, we would show a pretty ugly error
message. This change fixes that up to be more user friendly.
Theoretically this might catch other loading errors, but the new error
message should be generic enough to cover that.

Tried running with a missing or malformed config file. Saw the new
error.