pack build should fail when the specified image is already building

[sclevine]

Note: if we use a file to implement this, we should grab a lock on the file. Don't just use file presence to determine whether the image is locked. You should be able to kill -9 pack without leaving the image locked.

[natalieparellano]

Possible acceptance criteria:

When I run pack build some-image
And I run pack build some-image again while the previous invocation is still running
Then I should get a helpful error message

When I run pack build some-image
And I kill -9 pack (or the equivalent on Windows)
Then I should be able to run pack build some-image again without error