Ignore error if box not found when updating

[clintoncwolfe]

When performing a vagrant box update --box BOXNAME, the CLI command will exit with code 1 if the named box does not already exist. In our use case, we can ignore that error and happily continue, since our later vagrant up command will download the box if missing.

I considered explicitly listing boxes rather than ignoring an error, but that would introduce a performance penalty to every run.

Issues Resolved

Fixes #425

Check List

• All tests pass. See TESTING.md for details.
• New functionality includes testing.
• New functionality has been documented in the README if applicable.