Added more informative errors when trying to read netcdf3

[bnlawrence]

Description

The title says it all

Closes#263

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.59%. Comparing base (8550455) to head (4175eee).

Files with missing lines	Patch %	Lines
pyfive/high_level.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #267 +/- ##
==========================================
- Coverage 78.64% 78.59% -0.05% 
==========================================
Files 15 15 Lines 3488 3490 +2 Branches 555 555 ==========================================
Hits 2743 2743 - Misses 602 605 +3 + Partials 143 142 -1 

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[kmuehlbauer]

What happens if someone tries to open some other file (eg txt file)? Will there still be an informative error?

[bnlawrence]

The superblock is only looked for after the high-level has confirmed it is an HDF5 file.

[kmuehlbauer]

Thanks, I got a bit confused here 😬.

[bnlawrence]

(So, yes)

[davidhassell]

All good. Tested on the command line with various files (text, PP, netCDF3, HDF5)

[bnlawrence]

Argh. The test wasn't uploaded. I blame that ruff check. I'll push that up shortly.