Tools/scripts/sed_checked.sh: make more useful

• when sed expressions fail, report them, and the failing files, to aid developers in identifying the commits
• check whether a REINPLACE_CMD is partially effective, or totally ineffective, and adjust wording (please check vs. please FIX)
• use SED variable instead of /usr/bin/sed if set, for consistency.

portmgr@ hasn't raised objections other than performing power play,
and haven't responded with reasons to core@s question in
https://reviews.freebsd.org/D24240#592660 posed Sept 30 (17 days ago)

Approved by: portmgr@ (timeout, 200 days)
Differential Revision: https://reviews.freebsd.org/D24240