better feedback for malformed files and UI for tweaking/reimporting

Currently, if jalview fails to parse a file fully, it tends to raise an exception or throw an error message, and then attempt to parse the file using the PFAM sequence parser, which can lead to unexpected behaviour (random sequence lines appearing at the end of the alignment,etc). A more finely grained file format error handling system is needed to enable jalview to inform the user that a line was malformed, and prompt them as to whether they would like to:
1. parse the file using another/default parser
2. edit the file contents
3. cancel the import.

The presentation of options 1,2,3 should be adjusted according to the type/context of the input - e.g. whether the user was explicitly loading annotations/features/trees or whether the file can be displayed for editing in a text window.