[merb-core] Better error handling for Runner Adapter
[merb-core] Better error handling for Runner Adapter

Added a error handler for when the user tries to
run a script (using the Merb Runner Adapter) that
looks like a file but doesn't exist.

[#1231 state:resolved]
wycats/merb,wycats/merb,wycats/merb,merb/merb,merb/merb,merb/merb,wycats/merb,wycats/merb