Chained exception for invalid unit string?

[cdeil]

I was surprised how complex the exception is when trying to create a unit for an invalid string.

Would it be possible / make sense to simplify this and present a non-chained exception to the user in that case?

This is with Astropy 3.0.3 on Python 3.6, I did not check other versions.

>>> import astropy.units as u
>>> u.Unit('spam')
Traceback (most recent call last):
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/format/generic.py", line 482, in _do_parse
    return cls._parse_unit(s, detailed_exception=False)
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/format/generic.py", line 462, in _parse_unit
    raise ValueError()
ValueError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/format/generic.py", line 443, in _get_unit
    return cls._parse_unit(t.value)
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/format/generic.py", line 460, in _parse_unit
    s, did_you_mean(s, registry)))
ValueError: spam is not a valid unit. 

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/core.py", line 1784, in __call__
    return f.parse(s)
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/format/generic.py", line 469, in parse
    result = cls._do_parse(s, debug=debug)
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/format/generic.py", line 485, in _do_parse
    return cls._parser.parse(s, lexer=cls._lexer, debug=debug)
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/extern/ply/yacc.py", line 331, in parse
    return self.parseopt_notrack(input, lexer, debug, tracking, tokenfunc)
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/extern/ply/yacc.py", line 1061, in parseopt_notrack
    lookahead = get_token()     # Get the next token
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/extern/ply/lex.py", line 350, in token
    newtok = func(tok)
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/format/generic.py", line 151, in t_UNIT
    t.value = cls._get_unit(t)
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/format/generic.py", line 447, in _get_unit
    t.lexpos, str(e)))
ValueError: At col 0, spam is not a valid unit. 

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/deil/software/anaconda3/envs/gammapy-dev/lib/python3.6/site-packages/astropy/units/core.py", line 1798, in __call__
    raise ValueError(msg)
ValueError: 'spam' did not parse as unit: At col 0, spam is not a valid unit. 

[pllim]

Chaining is still seen in 3.1.dev22297. cc @mhvk

MHvK: Still there in 2021. Should really fix