Add informative error when converting symbolic dimensions to Unitful

[jkrumbiegel]

Fixes #79

[github-actions]

Benchmark Results

	main	bfd9d0e...	t[main]/t[bfd9d0e...]
Quantity/creation/Quantity(x)	3.4 ± 0 ns	3.1 ± 0.1 ns	1.1
Quantity/creation/Quantity(x, length=y)	3.4 ± 0 ns	3.4 ± 0 ns	1
Quantity/with_numbers/*real	3 ± 0.1 ns	3.4 ± 0 ns	0.882
Quantity/with_numbers/^int	11.9 ± 4.1 ns	11.9 ± 3.9 ns	1
Quantity/with_numbers/^int * real	12.2 ± 4.7 ns	12.7 ± 4.4 ns	0.961
Quantity/with_quantity/+y	7.1 ± 0 ns	7.7 ± 0.1 ns	0.922
Quantity/with_quantity//y	3.7 ± 0 ns	3.4 ± 0 ns	1.09
Quantity/with_self/dimension	1.7 ± 0 ns	1.7 ± 0 ns	1
Quantity/with_self/inv	3.4 ± 0 ns	3.4 ± 0 ns	1
Quantity/with_self/ustrip	1.7 ± 0 ns	1.7 ± 0 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.224 ± 0.17 ms	0.224 ± 0.17 ms	1
QuantityArray/broadcasting/multi_normal_array	0.076 ± 0.0023 ms	0.0788 ± 0.005 ms	0.964
QuantityArray/broadcasting/multi_quantity_array	0.254 ± 0.0012 ms	0.254 ± 0.0013 ms	1
QuantityArray/broadcasting/x^2_array_of_quantities	0.0432 ± 0.0054 ms	0.0436 ± 0.0054 ms	0.991
QuantityArray/broadcasting/x^2_normal_array	9 ± 1.7 μs	9 ± 1.6 μs	1
QuantityArray/broadcasting/x^2_quantity_array	10 ± 1.8 μs	9.6 ± 1.5 μs	1.04
QuantityArray/broadcasting/x^4_array_of_quantities	0.134 ± 0.0039 ms	0.137 ± 0.0063 ms	0.978
QuantityArray/broadcasting/x^4_normal_array	0.071 ± 0.003 ms	0.0687 ± 0.0007 ms	1.03
QuantityArray/broadcasting/x^4_quantity_array	0.107 ± 0.0015 ms	0.0916 ± 0.011 ms	1.16
time_to_load	0.206 ± 0.00076 s	0.206 ± 0.016 s	0.999

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[MilesCranmer]

Awesome. Thanks!

[MilesCranmer]

Do you need a release or on main for now until the next release is okay?

[jkrumbiegel]

I don't need it, just thought it'd be nice to have for others

[MilesCranmer]

Sounds good, ill do a release