String parser: template expression errors

[rtlayzell]

The string parser generates a new set of tokens based on the template expression after the $ or ${ because it's an entirely new lexical analysis, the location of the tokens isn't exactly correct, nor does it have access to the original source object which is a problem if we need to return errors (as they reference a source object).

Currently a workaround is provided: Instead of just returning the result of parse_expr, we construct a new error with the original string literal token and only the message from the result of parsing the expression.

Eventually, a proper fix will be needed to accurately display where the error occurred.

[Consider creating hierarchical errors rather than just flat lists]