More informative error message when function is missing

[liutiming]

Error: callr subprocess failed: argument "expr" is missing, with no default

I was a bit silly today and forgot to put the function for one function, but the error message was not indicative where the error may be.

Similar case when the target name is missing: Error: callr subprocess failed: attempt to use zero-length variable name

[wlandau]

Good point, informative errors should print as of a8cf19b.