Improve ENV/ARG error message

[apourchet]

The following Dockerfile caused an issue:

....
ARG VAR=x
ENV VAR $VAR

The error was the following:

failed to create build plan: failed to get dockerfile: failed to parse dockerfile: failed to create new directive (line 5): failed to parse ENV directive with args 'VAR': Missing space in single value ENV

[apourchet]

Turns out this bug isnt in Makisu, the value of VAR was passed in through --build-args, and had spaces.
Error message should be better.