Correctly log error message when pkg-install fails

[jburianek]

Original PR: iocage#1270

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If pkg-install fails, we should ignore blank lines when attempting to send back a helpful error message to the user.

This is because e.message can look something like this:

[b"pkg: No packages available to install matching 'fake-package' have been found in the repositories\n", b'', b'', b'']

Which leads to a very unhelpful error message like this:

Installing plugin packages:
  - fake-package... 
    - fake-package failed to install, retry #1
    - fake-package failed to install, retry #2
    - fake-package failed to install, retry #3

pkg error:
  - fake-package :

After this change the message is propagated back to the user:

Installing plugin packages:
  - fake-package... 
    - fake-package failed to install, retry #1
    - fake-package failed to install, retry #2
    - fake-package failed to install, retry #3

pkg error:
  - fake-package :pkg: No packages available to install matching 'fake-package' have been found in the repositories