fix(dump+reset): Check if workspace exists

[cwurm]

In dump and reset commands, checks if a specified workspace exists before trying to work with it. This makes sure the error message accurately reflects what the problem is.

E.g. right now:

$ deck dump -w Demo
Error: reading configuration from Kong: snis: Not found

After this PR:

$ deck dump -w Demo
Error: checking workspace 'Demo' in Kong: Not found

Uses the same code as is already in place for sync and diff.

[hbagdi]

We need to DRY some code out in cmd package to avoid such things from happening but that's another topic.
The error message that is thrown is incorrect but that should not be in scope for this PR. I've opened up #169 to track.