Throw error when instantiating process if one already exists for an account

[hswick]

An error should be returned if a process like a task giver, solver, or verifier is already running on account.

Example:

What currently happens:

🎲   $ start task -a 0
2018-09-10 18:10:00 [console] info: Task Giver initialized
🎲   $ start task -a 0
2018-09-10 18:20:21 [console] info: Task Giver initialized

What should happen:

🎲   $ start task -a 0
2018-09-10 18:10:00 [console] info: Task Giver initialized
🎲   $ start task -a 0
2018-09-10 18:20:21 [console] error: Task Giver already running for account 0.