Mappers to references elements don't show error messages if index build task fails

Scenario:

1. Open a valid workflow with "Map RNA-Seq Reads with TopHat" element.
2. Fill it with valid data.
3. Set the following parameters to the "Map RNA-Seq Reads with TopHat" element:
   • "Reference input type": "Sequence"
   • "Reference genome": url to some invalid data.
     Expected state: an index build task will be launched to generate index for tophat. It fails, there is an error in the log and on the dashboard.
     Current state: an index build task will be launched to generate index for tophat. It fails, there is an error in the log, but there are no errors on the dashboard.

Check all 6 mappers.