Fix inappropriate use of positive? as type predicate

A type predicate for a music function should accept all values. positive? only accepts numbers, and raises an error otherwise.

Input:

\magnifyMusic #'foo { c4 }

Before:

Parsing...ERROR: In procedure positive?: In procedure positive?: Wrong type argument in position 1: foo

After:

/tmp/frescobaldi-8kk_a0f4/tmpux8q3d2h/document.ly:3:15: error: wrong type for argument 1. Expecting positive number, found (quote foo) \magnifyMusic #'foo { c4 }