Adapt to new ChkTeX return values.

Adapt to new ChkTeX return values.

As of v. 1.7.7, chktex has four exit values. Only consider the program
failed with EXIT_FAILURE (1). This is backwards compatible to chktex
up to v. 1.7.5 and later patched versions included in TeXLive, where
there was the distinction EXIT_FAILURE (program failed) and EXIT_SUCCESS
(program successfully run, with or without something to report).

Note that ChkTeX v. 1.7.5 and 1.7.6 vanilla (as included in MikTeX) also
returned EXITE_FAILURE if ChkTeX found something to report.

We do not, and never did, support this case.

Fixes: #9989 (after ChkTeX 1.7.7. is released).