Information provided by fogbugz importer can be more user friendly if some operation fails

Steps:
1. This issue is related toJRA-15831.
2. Currently if user imports a fogbugz database having issues which are link to each other in suppose two ways(duplicate and related). And while importing if user just select one it and not select the other one.
3. We get a error message like

[23:28:30] FAILED: Unable to link issue from 4 to 2 with link named null 'java.lang.IllegalArgumentException: Name cannot be null.'
[23:28:30] Linking 'IJ-5' and 'IJ-3' as null
[23:28:30] FAILED: Unable to link issue from 5 to 3 with link named null 'java.lang.IllegalArgumentException: Name cannot be null.'
[23:28:30] Importing of Issue Links finished

Expected:
1. It will be great if we can provide a user friendly message specifying what exactly is missing. Currently if user is not aware of data, it only confuse.