Add support for ignoring errors via sushi-ignoreErrors.txt

[cmoesel]

Description: This PR allows authors to add a sushi-ignoreErrors.txt file to the root of their project (or input sub-directory) specifying errors that should be ignored. It follows the same file format as sushi-ignoreWarnings.txt:

Errors will be ignored if they completely match the contents of any line of the file (one line per warning, case-sensitive). Regular expressions can also be specified, one per line, indicated by starting and ending the line with /.

In addition, SUSHI will now indicate the number of ignored errors and/or warnings in its summary table (if applicable).

This feature was discussed on Zulip here: #IG creation > Cardinality of text element in resources.

Testing Instructions:

1. Create a simple SUSHI project with one or more errors (for example, missing required elements).
2. Run SUSHI and confirm that it logs these errors.
3. Add a sushi-ignoreErrors.txt file to the root of the project or in the input folder.
4. Add one or more lines to the file matching the error as indicated above.
5. Run SUSHI again and confirm that the errors were not logged and that the summary indicates the number of ignored errors.

Related Issue: N/A