Fix parsing issue (#106)

The document parser may get into problems when parsing emojis that extend past the internal buffering mechanism. If we switch to a SAX parser, this issue is avoided.

Improve error messaging on missing configurations (#102)

Some watch faces might be using user configuration nodes without declaring them. These are invalid watch faces, so the code was not treating this case properly and we were getting generic exceptions. This change adds a special error message when this error happens.