Specifying a new operation that already exists is not rejected

[Matthias Schoettle]

When adding an operation to a class, providing the signature of an operation that already exists is accepted without anything happening.

Steps to reproduce:

• Open the workflow concern
• Open the Workflow aspect
• Add operation to WorkflowExecutor with signature: + void run()
• Press enter

I think it would be more helpful to the user to reject this (i.e., the keyboard is not closed due to a validation error) allowing the user to update the signature or cancelling himself.

[Shihang Zhu]

Resolves #411: reject creating new operations that already existed by adding a check.

→ <<cset c01978c5aaa1>>

[Matthias Schoettle]

References #411: Rewords exception message.

→ <<cset 86d45ad11dcb>>

[Matthias Schoettle]

Merged in Issues/411-reject-existing-operation (pull request #84)

Resolves #411: avoid creating new operations that already existed

→ <<cset 256294127882>>