Refine QPDFParser error handling #1768

Job	Run time
Linux	7m 46s
Prebuild	2m 18s
AppImage	4m 39s
Windows (msvc, 64)	15m 4s
Windows (msvc, 32)	13m 56s
CodeCov	5m 35s
Windows (mingw, 64)	16m 9s
macOS	3m 16s
Windows (mingw, 32)	15m 9s
pikepdf	2m 32s
QuickJobs (build-fuzzer)	1m 47s
pikepdf (future)	2m 36s
Sanitizers	16m 19s
QuickJobs (pkg-test)	3m 18s
QuickJobs (build-linux32)	3m 25s
Zopfli	11m 57s
QuickJobs (test-alt-zlib)	3m 44s
QuickJobs (test-unsigned-char)	3m 17s
QuickJobs (test-c++-next)	3m 34s
MergeArtifacts	12s
	2h 16m 33s

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