Add a new program option: -dwarning-as-error

Add a new program option: -dwarning-as-error

(Thanks to Han-Wen and Jan for the initial idea)

- If a valid stencil expression is found, but is
undefined in the current backend, issue a warning
for every occurrence.

- The option -dwarning-as-error will cause LilyPond
to output an error and exit upon the first warning
encountered.