Exit with non-zero error code if specified env file does not exist

[QuantumGhost]

Honcho version: 1.0.1
Scenario: run honcho -e $config run xxx with
Expected behavior: Honcho exits with error.
Actual behavior: honcho runs without printing any error message and exits with error code 0.

I was bitten by this issue and it took me about half an hour to figure out why my app is not behaving as expected.
I think it's much better to exit with an error if env file specified does not exist.