flag: return a consistent parse error if the flag value is invalid
Return a consistently formatted error string that reports either
a parse error or a range error.
Before:
invalid boolean value "3" for -debug: strconv.ParseBool: parsing "3": invalid syntax
After:
invalid boolean value "3" for -debug: parse error
Fixes #26822
Change-Id: I60992bf23da32a4c0cf32472a8af486a3c9674ad
Reviewed-on: https://go-review.googlesource.com/c/143257 Reviewed-by: Ian Lance Taylor <iant@golang.org>