bec silently fails if config file cannot be found

[jesperes]

To reproduce:

07:38 $ _build/default/bin/bec  -c foobar ; echo $?
Could not read config file "foobar": (enoent).
07:38:59.306 [error] Error while reading config file "foobar" (enoent).
0

This was discovered after a bug in our pipelines caused the config file to be absent, but bec just silently ignored it and terminated successfully.