Update Debugging chapter of Lisp manual.

Update Debugging chapter of Lisp manual.

* doc/emacs/debugging.texi (Debugging): Copyedits.  Describe testcover, ERT.
(Error Debugging): Note that debug-ignored-errors overrides list
values of debug-on-error too.  Add xref to Signaling Errors.  Note
that debug-on-signal is not customizable.  Mention
condition-case-unless-debug.
(Compilation Errors): Node deleted.

* doc/emacs/compile.texi (Compiler Errors): Move a paragraph here from
deleted node Compilation Errors.