Remove unused error classes.

[bdice]

Description

Removes unused error classes from signac 2.0.

Motivation and Context

signac 2.0 API cleanup.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[codecov]

Codecov Report

Merging #595 (0ec5108) into next (579800d) will decrease coverage by 0.75%.
The diff coverage is 92.30%.

❗ Current head 0ec5108 differs from pull request most recent head e0b9217. Consider uploading reports for the commit e0b9217 to get more accurate results

@@            Coverage Diff             @@
##             next     #595      +/-   ##
==========================================
- Coverage   84.76%   84.00%   -0.76%     
==========================================
  Files          52       53       +1     
  Lines        5264     5327      +63     
  Branches      964      980      +16     
==========================================
+ Hits         4462     4475      +13     
- Misses        587      635      +48     
- Partials      215      217       +2     

Impacted Files	Coverage Δ
signac/__main__.py	75.17% <0.00%> (ø)
signac/contrib/utility.py	51.88% <ø> (-16.61%)	⬇️
signac/contrib/project.py	86.22% <100.00%> (ø)
signac/errors.py	92.30% <100.00%> (ø)
signac/contrib/linked_view.py	83.91% <0.00%> (-4.24%)	⬇️
signac/contrib/schema.py	95.48% <0.00%> (-0.76%)	⬇️
signac/contrib/collection.py	83.82% <0.00%> (-0.61%)	⬇️
... and 1 more

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[bdice]

@csadorf Sorry, I hadn't rebased this branch correctly at the time you reviewed. I fixed that now and will merge once tests pass.