fix: import-time side effect on errors

A subtle module mismatch occurs when importing:

    from marconiclient.transport import errors

This actually brings in marconiclient.errors, rather than the expected
marconiclient.transport.errors. This patch addresses the issue, which
was caused by import-time side effects in
marconiclient.transport.__init__.

Change-Id: I2209193fc81099b1af3e0fc174b254cdfcfc619a
Merge "fix: import-time side effect on errors"