[llvm-cov] Keep the detailed error message in CoverageMappingIterator

[whentojump]

Currently errors returned from CoverageMappingIterator will have their message part discarded. This commit copies that message when constructing the new CoverageMapError object, and let it displayed at the end caller (llvm-cov).

Related: #65264

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[whentojump]

From discussions in #65264:

It seems that most message are untested?

I can add more tests for this end if that's desirable. An example would look like

#!/bin/bash

rm main *.profraw *.profdata

cat << EOF > main.c
int main(void) {
  if (1)
    return 0;
}
EOF

clang -fprofile-instr-generate -fcoverage-mapping main.c -o main
llvm-cov convert-for-testing main -o main.covmapping

# Manually corrupt the region kind
cat main.covmapping | bbe -e 's|\x20|\x48|' > main_malformed.covmapping

LLVM_PROFILE_FILE="main.profraw" ./main
llvm-profdata merge main.profraw -o main.profdata
llvm-cov show main_malformed.covmapping -instr-profile main.profdata

The result should ideally be

error: failed to load coverage: 'main_malformed.covmapping': malformed coverage data: region kind is incorrect

rather than

error: failed to load coverage: 'main_malformed.covmapping': malformed coverage data

The malformed covmapping file is generated by manipulating good ones with bbe. If this way is acceptable, I am happy to rewrite such scripts in Clang/LLVM style test suites.

[whentojump]

Mistakenly pressed some buttons, sorry for the mess...

@MaskRay Could you please take a look? Thanks!