Improve error message when cannot find a named HDU in a fits file

[gsleap]

Example from @d3v-null:

03_mwalib.py test_files/1120082744/1120082744_DipAmps.metafits   
Traceback (most recent call last):
  File "/demo/03_mwalib.py", line 62, in <module>
    main()
  File "/demo/03_mwalib.py", line 48, in main
    ctx = MetafitsContext(metafits)
          ^^^^^^^^^^^^^^^^^^^^^^^^^
mwalib.MwalibError: src/metafits_context/mod.rs:682
test_files/1120082744/1120082744_DipAmps.metafits HDU 9999: Fits error: FitsError { status: 301, message: "illegal HDU number" }

In this case the actual error was that the TILEDATA hdu was present but did not have:
EXTNAME = 'TILEDATA' / extension name

A better error message would have saved some troubleshooting time and been clearer- e.g.:

Cannot locate 'TILEDATA' HDU in FITS file: xxxxx.fits

[gsleap]

NOTE: test file is from v1.3 of hyperdrive in test_files/1120082744/1120082744_DipAmps.metafits