Description of problem:
When adding a new mount point and an invalid mount point is entered (for example "home"), anaconda doesn't allow you to move forward nor back. 

Version-Release number of selected component (if applicable):
anaconda 18.10

How reproducible:
always

Steps to Reproduce:
1. start installation in vnc mode
2. select Installation destination
3. select all available disks
4. select "Let me customize..."
5. delete all existing partitions
6. click on "+" to add new partition
7. enter Mount Point: "home", Desired Capacity: 500
8. click on "Add mount point" - nothing happens, no error message
9. click on "Cancel" - nothing happens, dialog window doesn't close

Actual results:
No warning/error message is shown for invalid mount point, unable to close the dialog window.

Expected results:
User is informed if specified mount point cannot be created, clicking on Cancel close the dialog window.