Merge branch 'SLADE_textures' into 'next'

Treating " as whitespace in TEXTURES (and animdefs too i guess)

SLADE adds this character in its TEXTURES editor and makes SRB2 cry unless the lump is manually edited; this just treats it as whitespace so we don't have to think too hard about it.

Also, two I_Errors now refer to the correct lump name.

See merge request !122