Improve management and surfacing of errors when adding conversions

[jscholes]

At present, errors are raised when a conversion is added to the queue. But given the new intended workflow whereby options are chosen per conversion, this doesn't make much sense. For example:

• The error message informing a user that a filetype is not supported is only shown once an output format has been chosen. The error should be shown upfront.
• An error message is shown when a file has already been added, but this should be extended so that users can add multiple instances of the same file with different conversion options.
• When adding multiple files from a directory, errors are detected but not surfaced for the user. A method of summarising any errors should be provided which a user can review before choosing their conversion options.

[jscholes]

The application is not actively maintained or supported. Pre-built versions, and the corresponding project page, are no longer available. As such, this will not likely be worked on.