Stop ignoring most scan chain validation errors

Stop ignoring most scan chain validation errors

Among other things this causes startup errors to kick in the
fallback "reset harder" logic during server startup.  Comments
are also updated a bit, explaining what the various error paths
signify (in at least my observation).

There's one class of validation error that we can still plausibly
ignore:  when wrong IDCODE values are observed.

This change seems to have helped make an OMAP5912 behave much
more reliably.  There's still some post-reset flakiness, but
it's unrelated to scan verification.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>