All illegal function identifiers should result in parse errors

[mistydemeo]

Illegal function identifiers (for example, identifiers beginning with a number) are only flagged for the first function call in a series; otherwise the function call is simply skipped during parsing.

For example, this fails as expected:

$ xg 1sub
Unable to parse provided program: Expected W:(abcd...,abcd...) (at char 0), (line:1, col:1)

However, this works, with the invalid expressions skipped:

$ xg 'sub 2sub sub'