Fix error handling for array parsing

[natecook1000]

Addresses an error introduced when rearranging error handling in array parsers. The validation of count being non-negative was dropped at some point along the way; this change brings that check back and switches the Array(parsing:count:parser:) initializer to throw ThrownParsingError instead of having a generic error type parameter.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary