Raise NotImplementedError when parsing with output-only format

[mhvk]

EDIT: Really, just resolving over-eager exception catching...

Previously,

u.Unit('m', format='unicode')
...
ValueError: 'm' did not parse as unicode unit: Can not parse Unicode If this is meant to be a custom unit, define it with 'u.def_unit'. To have it recognized inside a file reader or other code, enable it with 'u.add_enabled_units'. For details, see https://docs.astropy.org/en/latest/units/combining_and_defining.html

Now:

NotImplementedError: Can not parse with Unicode format

fixes #11828

milestone'd 5.0, since really this is just a change in error type.

cc @olebole, @pllim

[pllim]

Seems uncontroversial to me but I'll leave it open for Ole's review.

[mhvk]

Am merging since it will help get me out of rebase problems in #11830...

[mhvk]

@olebole - input is still welcome! If you see problems, I'll fix separately...

[olebole]

No problems, however my point was that the message with an unknown input format is confusing:

>>> u.Unit('Ω', format='foo')
[…]
ValueError: Unknown format 'foo'.  Valid formatter names are: [base, generic, unscaled, cds, console, fits, latex, latex_inline, ogip, unicode, vounit]

This error says that unicode is a valid format(ter) here, which then does not work. IMO the list shown in this ValueError should contain only the formatters that really can parse any input.

[mhvk]

Hmm, that is actually fairly tricky, since the unknown format error is raised in units.format.get_format() and that doesn't know a parser is desired. But maybe #11831 is at least a step on the way?