Tweak error message to suggest adding an owned bound

[jruderman]

In error messages, consider changing

use `owned` bound

to

add `owned` bound

as a subtle hint that you can have more than one bound. (cf #2991, #2993)

[nikomatsakis]

Not critical for 0.6; removing milestone

[graydon]

sure. easy fix. patch in the queue, #6340