Disallow defining a duplicate decision point in Decision Points table

[zackcl]

Is your feature request related to a problem? Please describe.
Currently, it is allowed to add a decision point that already exists in Decision Points table. We should not allow this in the Add/Edit Decision Points modal similar to how we disallow duplicate condition name in Add/Edit Condition modal.

Describe the solution you'd like
We should probably display the validation error message below the Site and Target fields like this (when invalid):