Clean up docs about error handling

[davidism]

There are many pages in the docs that have different overlapping information about error handlers:

• https://flask.palletsprojects.com/en/1.1.x/errorhandling/#error-handlers
• https://flask.palletsprojects.com/en/1.1.x/blueprints/#error-handlers
• https://flask.palletsprojects.com/en/1.1.x/patterns/errorpages/
• https://flask.palletsprojects.com/en/1.1.x/patterns/apierrors/

These need to be consolidated, probably into the errorhandling.rst page, with a link to the blueprints page section for blueprint-specific information.

Also, in #3572 @mikeckennedy brought to our attention that the docs about 404 and 500 handlers for blueprints needs clarification with better examples and explanation of why they don't work in all cases.

[jackwardell]

would be happy to have a crack at this if desired

[davidism]

No need to ask, if there's no PR linked to an issue just go for it!

[jackwardell]

is there a reason why register_error_handler has an underscore between "error" and "handler" when errorhandler does not?

[davidism]

It came later and I wanted to use snake case going forward.