Better output when makepot fails due to missing PHP cli

[jnachtigall]

I switched over to a new laptop and made development based on docker. Then I did not notice that makepot did not work anymore: It did run, but just outputted: undefined.

Took me some time to notice, then some time to find the culprit: PHP cli was not installed locally (only in docker).

The https://github.com/cedaro/grunt-wp-i18n#requirements say so: PHP CLI must be in your system path. but still I think the makepot output could be a bit more helpful and instead of undefined print something like Could not find php in your system path - is PHP CLI installed locally?