Check for nonexistent names in acset macro

[slwu89]

Addresses #55; this exact issue recently came up when introducing a new user to acsets, so it seemed timely to address.

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (f520029) 91.05% compared to head (ff51a74) 90.88%.

Files	Patch %	Lines
src/DenseACSets.jl	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #110 +/- ## ========================================== - Coverage 91.05% 90.88% -0.18% ========================================== Files 22 22 Lines 2136 2140 +4 ========================================== Hits 1945 1945 - Misses 191 195 +4 ```

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[slwu89]

thanks @jpfairbanks, good to merge?

[epatters]

Thanks Sean!