Create validation for document types with error handling

[oakal510]

• This PR is for added validation checks for document types with error handling for failing to provide requisite document type details relating to icon, description, extension, and URL for a given document type in an app. Existing tests were updated to account for the validation and new tests were created to ensure proper validation.
• This PR will ensure that if a user does not provide valid document type details, the user will receive clear error messages.

Fixes #1770

PR Checklist:

• [x] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct