Improve `IncompleteParseError` message

[jelling]

Is your feature request related to a problem? Please describe.

If a user creates a guidance program with a space in between the handlebars and the command like the space prior to "gen" here:

{{ gen "out" n=3 max_tokens=75}}

The error returned is:

IncompleteParseError: Rule 'template' matched in its entirety, but it didn't consume all the text. The non-matching portion of the text begins with...

Describe the solution you'd like

A clearer error message like "Space detected between {{ and gen..." would be more helpful.