External commands lead to cryptic failure instead of readable error message

As discussed in #32 (closed), external commands are not supported during parsing as $PATH is invalid. This sounds reasonable, though an explicit error message would be appropriate too.

I'm thinking something with a tag like external-commands:

bluespec-git E: References external commands (commands [pacman, cut, awk] in PKGBUILD)

If necessary, the message can then be looked up in the ArchWiki (once it's added there)