Show a more specific error message when download failed due to missing GPG key

[eloquence]

#1059 improved the error handling for message/reply downloads when a required GPG key is missing, but file download errors still simply get reported as "The file download failed. Please try again" in that case, which is misleading -- trying again will have no effect, and the download itself was not the problem. We also don't support adding multiple keys yet, so there is not always a remediation path.

The language for this case proposed by @ninavizz in #140 (comment) can be seen in the screenshot below (the error bar at the top): "Cannot decrypt file. The Submission Key used for the file is missing."

[zenmonkeykstop]

This should go in the backlog:

• the longer folks are using the workstation, the more chance they rotate submission keys, so it's likely to come up
• that said, the UI above doesn't surface which file is b0rked, so we'd need to think about UX improvements in that vein first
• a decryption-related exception is thrown, so UI is probably the more challenging part here.