Merge pull request dronekit#587 from dronekit/tcr-please

Catch and display message and attribute errors, then continue.

Merge pull request dronekit#598 from dronekit/rroche/bump-2.4.0

bump to 2.4.0

Merge pull request dronekit#589 from dronekit/v2.3-versioning

bump version to 2.3.0

Merge pull request dronekit#575 from dronekit/rroche/version_2.2.0

bump release to 2.2.0

Merge pull request dronekit#557 from dronekit/release_branch_2_1_0_up…

…date

Update setup.py and doc.conf to release 2.1.0