Diff command should imply validation

[dgreene-r7]

Description

Currently, running the convection diff command does not validate the template for a given stack and can suppress errors that come up when converging. It would be nice if the diff command could also validate the stack that you're diffing.

Expected behavior

Diffed stacks are also validated.

Observed behavior

Diffed stacks are not validated. 😛

[erran-r7]

I'm happy with having this as the default behaviour. @rdickey @bcall-r7 @cdoughty-r7 thoughts?