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>

src/jtag/jtag.h

@@ -689,6 +689,7 @@ extern void jtag_sleep(uint32_t us);
 #define ERROR_JTAG_DEVICE_ERROR      (-107)
 #define ERROR_JTAG_STATE_INVALID     (-108)
 #define ERROR_JTAG_TRANSITION_INVALID (-109)
+#define ERROR_JTAG_INIT_SOFT_FAIL    (-110)
 
 /**
  * jtag_add_dr_out() is a version of jtag_add_dr_scan() which