Make the page loading error message configurable (similar to loadingMessage)

[oiva]

The default page loading message can be overridden in mobileinit but the default error message "Error Loading Page" is hard coded in the error handler.

It would be nice to show a localized error message when a page cannot be loaded.

This issue is similar to #363 but easier to fix. It's also easier for a developer to change the text instead of writing an error handler function.

[scottjehl]

Added pageLoadErrorMessage option. Defaults to "Error Loading Page". Documentation included. Closed by 4a45d8d and Closed by 4a45d8d