Fix up diagnostic.pm’s backtrace for multiline msgs

Fix up diagnostic.pm’s backtrace for multiline msgs

I hope you agree this backtrace does not look very good:

$ ./perl -Ilib -Mdiagnostics -e '
  sub foo { bar () }
  sub bar { baz() }
  sub baz {
  die "Attempt to reload foo aborted.\nCompilation failed in require"} foo
'
Attempt to reload foo aborted (#1)
    (F) You tried to load a file with use or require that failed to
    compile once already.  Perl will not try to compile this file again
    unless you delete its entry from %INC.  See "require" in perlfunc and
    "%INC" in perlvar.

Compilation failed in require at -e line 1 (#2)
    (F) Perl could not compile a file specified in a require statement.
    Perl uses this generic message when none of the errors that it
    encountered were severe enough to halt compilation immediately.

Uncaught exception from user code:
	Attempt to reload foo aborted.
Compilation failed in require at -e line 1.
	main::baz() called at -e line 1
	main::bar() called at -e line 1
	main::foo() called at -e line 1