Reconcile all Carvel tools to behave identically when no arguments are given

**Describe the problem/challenge you have**
Reported first as: vmware-tanzu/carvel-kbld#184

For each command-line tool in the Carvel suite, they behave differently from others when no arguments are given.

| tool | displays help | displays error message | exit status |
| --- | --- | --- | --- |
| imgpkg | ❌ | ✅ | 1 |
| kapp | ✅ | ✅ | 1 |
| kbld | ❌ | ❌ (displays "Succeeded") | 0 |
| kwt | ✅ | ✅ | 1 |
| vendir | ❌ | ✅ | 1 |
| ytt | ❌ | ❌ | 0 |

**Describe the solution you'd like**
For all the tools to behave identically, unless there's a clear reason to be different.

Seems like the most useful behavior would be to:
- display the help text,
- report an error message, and
- exit with a non-zero exit status.

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

Start by comparing the no-argument behavior of imgpkg, kapp, kbld, kwt, vendir, and ytt, using the table as the expected baseline. The work is done when each tool displays help, reports an error, and exits with a non-zero status, unless a documented reason justifies different behavior.

Written by the indexing model from the issue text.