Fix error messages for `min` and `max`
Fix error messages for `min` and `max`

This PR fixes the error messages for the `min` and `max` functions
when they're passed a non-numeric value.

Fixes #1559
Spec sass/spec#612

am11/libsass,am11/libsass,am11/libsass,am11/libsass,am11/libsass