Improve exec hook output/behavior, fixes #1818, fixes #1765

[rfay]

The Problem/Issue/Bug:

#1818 points out that there are cases where an exec task fails and no output is provided
#1765 requests clearer feedback about the problem when an exec task fails, instead of giving a bunch of docker-compose gobbledygook.

How this PR Solves The Problem:

• Show output of hook task whether or not it fails (exec-host and exec hook command output should be displayed even if there's an error #1818)
• Don't show a bunch of docker-compose nonsense
• Add a warning explaining that this is configuration that failed (Provide clearer feedback when a hook's "exec" or "exec-host" task fails #1765)
• A failed task does not fail the entire operation.

Manual Testing Instructions:

Try this in your config.yaml:

hooks:
 pre-start:
   - exec-host:  ls junker100
 post-start:
   - exec: ls junker99

and do a ddev start

• The pre-start hook should fail with appropriate information
• The post-start ls junker99 should fail with appropriate information

Automated Testing Overview:

No new testing is added here.

Related Issue Link(s):

#1818
#1765

Release/Deployment notes:

Add to release notes:

• Failure of hook task doesn't cause failure of start (or anything else)

[rfay]

Artifacts are at https://circleci.com/gh/drud/ddev/18254#artifacts/containers/0

[greggles]

This is a very nice improvement!

[rfay]

Rebased and retested; I think this is ready to go when tests pass.