Feedback for unsupported export formats

[UWinch]

Is your feature request related to a problem? Please describe. When the user exports annotations from the annotation page, the dropdown for available export formats also includes those which might not support the existing annotations. In case of an unsupported format, the user is re-directed to a document opening dialog. The user might conclude that this is a bug.

Describe the solution you'd like Show a feedback message to the user explaining that the chosen format is not supported.

Describe alternatives you've considered Do not list unsupported formats in the dropdown. However, this is hard to know at this point.

[reckart]

I could imagine a two-step approach:

• FormatSupport would need to be extended to allow checking if a particular format is compatible with the current layer configuration of the project - however, this is not sufficient since the project might contain annotations which do not follow the current configuration. For example, if a user has created two overlapping named entity annotations and later change the configuration of the named entity layer to prevent the creation of overlapping annotations - several CoNLL formats can export named entities, but only if they do not overlap.
• so a second measure has to be to validate the project (or at least the file being exported) according to the current layer configuration (we already have the code to do that)

I believe this two-step approach would be preferable e.g. to implementing the complete validation logic in the format support because it provides a decent separation of concerns and allows to re-use the existing validation code.