Warn the user when unrecognized image meta is dropped.

[pivotaljohn]

Describe the problem/challenge you have
When image meta is being parsed from an upstream imgpkg lock file,
if the structure of the meta is unknown to kbld (e.g. the meta was created by a newer version of kbld than the one being used to deploy),
then that meta is discarded without warning.

Describe the solution you'd like
While parsing meta, if one is unrecognized:

• with normal output, give one general warning (e.g. "There was some resolution history that was unrecognized and therefore discarded. This had no effect on successfully resolving image references. Run again with --debug for details.")
• if the user supplies the --debug flag (which doesn't exist at the time of writing), then report each unrecognized meta.

Anything else you would like to add:

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