[toolchains] Editor should not allow for duplicated connections

Our toolchain editor currently allows the user to draw duplicated connections from a block to another block, while this should be illegal.

Steps to reproduce:

1. For any toolchain
2. On the editor, create a new connection duplicating an existing one
3. Type a name and hit the save button

Results:

Error for duplicated connection, with cryptic message.

Expected:

Either a more explicit error message indicating which connections are duplicated or a smarter toolchain editor that does not allow for this.