Improve error messages for Error Tracking settings form

What does this MR do?

Add validation for api_url path to ensure that it is in the form: /api/0/projects/{sentry_organization_slug}/{sentry_project_slug}/ so that the full api_url is of the form: scheme://<sentry-host>/api/0/projects/{sentry_organization_slug}/{sentry_project_slug}/.

The host part of the URL is already being validated by the url_validator.

Also, improve the wording of the error messages.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57358

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer