text/scanner: don't crash when calling TokenText in error handler

text/scanner: don't crash when calling TokenText in error handler

Make sure Scanner.tokEnd is set before we call Scanner.Error
and update documentation accordingly.
(Until now tokEnd was only set before returning from Scan,
so a call to TokenText during error handling may have crashed.)

While at it, tighten a check in Scanner.TokenText to ensure
Scanner.tokEnd >= Scanner.tokPos if we have a token.

Also, silence error messages to Stderr in unrelated TestIllegalExponent.

Fixes #29723.

Change-Id: Ia97beeae91eaf9e0ed3dada0a806f1f7122461cc
Reviewed-on: https://go-review.googlesource.com/c/157819
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>