Subject write errors crash the program #339

Note on provenance: This issue was found by Claude Opus 5 while reviewing #320. The reproductions and output shown below were produced by building and running the code, but I have only briefly reviewed the analysis myself and I am not an expert in this area. Please review it critically.

Impact

Two ordinary situations kill the process instead of returning an error the caller can handle: writing subject metadata where a field is an empty string, and writing a subject to a file that already has one.

A caller that wraps the call in a normal try/catch still terminates, because H5::Exception does not derive from std::exception:

try {
  nwbfile->initialize(uuid, desc, coll, t, t, std::optional(spec));
} catch (const std::exception& e) {
  // never reached, the process terminates
}

An acquisition program losing a recording session to a blank text field is a bad failure mode for a library whose job is to not lose data.

What happens

The scalar overload of HDF5IO::createStringDataSet (src/io/hdf5/HDF5IO.cpp:1254) has no try/catch, so an HDF5 failure travels straight out through Subject::initialize and NWBFile::initialize. The vector overload just below it already catches and returns Status::Failure.

An empty string field. SubjectSpec fields are std::optional<std::string>, so an empty string is a value that is set, not a value that is absent. Writing subjectSpec.sex = "" is a natural way to say "this value is not known", and it is easy to reach when subject metadata comes from a config file, a GUI text box, or a database column that returned nothing. All ten branches in Subject::initialize test only has_value() (src/nwb/file/Subject.cpp:55 and following), so the empty string reaches HDF5 as a string type of size zero, which HDF5 rejects.

A subject that already exists. Calling Subject::initialize a second time, or calling it on a file opened in append mode that already contains /general/subject, tries to create datasets that are already there. NWBFile::initialize guards this with an objectExists check (src/nwb/NWBFile.cpp:106), so it reaches users through the Subject class directly, which #320 makes public.

empty string       H5::Exception  func=StrType::setSize        detail=H5Tset_size failed
already exists     H5::Exception  func=H5File::createDataSet   detail=H5Dcreate2 failed

Suggested direction

Catch H5::Exception in the scalar createStringDataSet and return Status::Failure, matching the vector overload. That closes both triggers and protects every other caller of that function.

Then skip fields whose value is empty, the way NWBFile::createFileStructure already handles its own optional string dataset at src/nwb/NWBFile.cpp:197, so an unknown value is simply not written.

if (subjectSpec.sex.has_value() && !subjectSpec.sex->empty()) {
  ...
}