strconv: ParseBool errors are verbose

[alandonovan]

Failure to parse a boolean flag results in this error:

https://play.golang.org/p/cBBLtiFyWhY:
invalid boolean value "garbage" for -b: strconv.ParseBool: parsing "garbage": invalid syntax

Room for improvement:

• display the offending value only once
• don't mention strconv.ParseBool, as it is an internal detail.
• state what values are allowed, perhaps?

The other strconv.Parse* functions are similarly verbose. I've gotten into the habit of throwing away their errors and writing my own. e.g. "invalid int: %q".

[agnivade]

/cc @robpike

[Puthranps]

If this proposal gets approved I can work on it if possible