fix error handling when failing to list magefiles

[zbindenren]

This pull request fixes error handling. It changes the message from:

No .go files marked with the mage build tag in this directory.

to:

Error determining list of magefiles: failed to list non-mage gofiles: exec: "go": executable file not found in $PATH

when for example the go binary is not found in PATH.

[natefinch]

Awesome, thank you!