fix exit status in QUIET mode

Without the parentheses, this was causing the line to always execute false and
exit with status 1.
in QUIET mode, pipe pdflatex output to /dev/null

This suppresses some annoying messages that otherwise leak through, including
"This is pdfTeX..." and "entering extended mode".
use ERRFILTER everywhere PDFLATEX is run

This is probably overkill for what I'm trying to accomplish, which is
suppressing the "leaked" output from pdflatex. It would suffice to just use ">
/dev/null" in these other invocations because they won't be run if the first
pdflatex invocation encounters an error. It is sufficient to do this, though,
and a little easier to write.
fix error regex

[[:digit:]] as a whole is equivalent to \d, so the + needs to go outside of
the whole entity.