Show error message upon error opening log file

[xrobin]

Summary

The CifCheck command fails silently when it cannot write to the *-diag.log file (eg if $PWD is read-only). ofstream::open() is unchecked and validation errors are lost. With no output and an exit code of 0, it looks exactly like the input file passed validation.

Steps to reproduce:

#  Create the test directory and a minimally invalid CIF file:
mkdir -p /tmp/test_mmcif
cat > /tmp/test_mmcif/invalid.cif << 'EOF'
data_test
_invalid_category.field value
EOF
# Check validation
CifCheck -f invalid.cif -dictSdb /path/to/mmcif_pdbx_v50.sdb
# Creates an output
cat invalid.cif-diag.log 
# ERROR - In block "test", category "invalid_category" is not defined in the reference file
# ERROR - In block "test", in category "invalid_category", item "field" is not defined in the reference file
# ERROR - In block "test", category "invalid_category" is not defined in the reference file
# ERROR - In block "test", in category "invalid_category", item "field" is not defined in the reference file

# Remove write permission on the directory:
rm invalid.cif-diag.log
chmod a-w /tmp/test_mmcif

# Run CifCheck again
CifCheck -f invalid.cif -dictSdb /path/to/mmcif_pdbx_v50.sdb
# Appears successful: no error displayed on stderr, no diagnostic file:
ls invalid.cif-diag.log
# ls: cannot access 'invalid.cif-diag.log': No such file or directory

Expected output

There should be some indication that the file invalid.cif-diag.log is absent not because the cif file is valid, but because CifCheck it failed to create it.

Changes

With this commit, a FileException is thrown in CifFile::DataChecking() if the diagnostic log cannot be opened or written.