Safe no-op for \advance when missing definition

[dginev]

Fixes #878

It upgrades the fatal message from the example:

Fatal:misdefined:LaTeXML::Package::Pool::__ANON__ Can't locate method 'valueOf' via 'missing'
	at /home/deyan/perl5/lib/perl5/LaTeXML/Package/TeX.pool.ltxml line 1221, <$IN> line 3.
	In Core::Definition::Primitive[\advance ... from TeX.pool.ltxml line 1221
	 <= Core::Stomach[@0x55f4b4414ec8] <= Core::Definition::Primitive[Begin] <= Core::Stomach[@0x55f4b4414ec8]
3 errors; 1 fatal error; 1 undefined macro[\AtBeginPart]

To a less critical error report:

Conversion complete: 3 errors; 1 undefined macro[\AtBeginPart].

Which is still sufficiently informative about the root cause. Feel free to review & merge @brucemiller

[brucemiller]

seems a good idea.