Description of problem:
Pressing the Import button when no file is specified returns a dialog indicating:
"There was an error uploading the file: Bad Request (400)"
A user generally wouldn't know what an error 400 is. A good UI would not let the user press the button at all.

Version-Release number of selected component (if applicable):
3.0

How reproducible:
Easy always

Steps to Reproduce:
1. Login as admin
2. Go to Administration, Translation Memory
3. Create a new TM if needed, press Import
4. Press the Import button

Actual results:
There was an error uploading the file: Bad Request (400)

Expected results:
No available button to upload (or at least a better error message). Uploading source documents has a similar disabled button.