Explain cause when error uploading on Artifact

When uploading of an Artifact to the Mender Server fails, there are a couple of generic error messages that are shown.
This is causing some diagnostics headache for users.

It does not seem like we explain that the Artifact format version is not supported either, in these cases.
I tested this with Mender Server 1.0.1 and uploaded an Artifact with format version 2. I just got generic error messages (see attached).

Acceptance criteria

• When an Artifact fails to upload, the cause is shown, both in the UI and API
• If the Artifact format version is not supported, the error message includes the Artifact formats version this Mender Server supports and the Artifact format version of the Artifact uploaded
• If the checksum mismatches, this is indicated and the calculated as well as the checksum in the Artifact are both shown