Issue #95: Add Portal/URL validation error messages.
Issue #95: Improve error message utility.

* Allow setting fallback error message.

* Add support for Portal user authorization errors.

* Use available error response message as fallback.

* Clean up.
Issue #95: Show error message in URL/Portal checkers.
Issue #95: Display Portal authorization errors prominently.
Issue #95: Short-circuit Portal URL adjustment if Portal error response contains message.
Show URL validation progress indicator when it has truly started.
Increase the Portal validation request timeout.
Stop validation when URL changes.
Merge branch '#95/show-known-input-checker-error-messages' into develop
Issue #101: Update AttributeTable & Edit exclude layer lists properly.
Clean up.

* Consolidate property-change flags in commitProperties().

* Clean up logic that selects included layers.

* Remove unused variable.

* Add missing semicolon.
Merge branch '#101/fix-odd-exclude-layer-list-scrolling-behavior' into develop