Issues related to adding/editing/deleting flag priorities

First issue:

When we want to save priority with non-unique name we get an error page: 

Reproduction steps:

1. Go to the System Administration -> Advanced Administration -> Manage Priorities
2. Click Add Priority
3. Enter name that already exists
4. Enter style and rank values
5. Click Save Priority

Actual: Error page appears

Expected behavior: User should get validation message that this name already exists

Second issue:

When we want to save priority with empty style we get an error page: 

Reproduction steps:

1. Go to the System Administration -> Advanced Administration -> Manage Priorities
2. Click Add Priority
3. Enter name and rank values
4. Leave style value empty
5. Click Save Priority

Actual: Error page appears

Expected behavior: User should get validation message that style value is required

Acceptance criteria:

• issues mentioned above should be fixed