Throw a proper error if negative funds are given to the connect endpoint

[LefterisJP]

Problem Definition

If you provide negative funds to the connect endpoint like below:

http PUT http://localhost:5001/api/1/connections/0x0f114A1E9Db192502E7856309cc899952b3db1ED funds=-10

then an internal server error is thrown

Traceback (most recent call last):
  File "/home/lefteris/.virtualenvs/raiden/lib/python3.6/site-packages/flask/app.py", line 1813, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/lefteris/.virtualenvs/raiden/lib/python3.6/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/lefteris/.virtualenvs/raiden/lib/python3.6/site-packages/flask_restful/__init__.py", line 480, in wrapper
    resp = resource(*args, **kwargs)
  File "/home/lefteris/.virtualenvs/raiden/lib/python3.6/site-packages/flask/views.py", line 88, in view
    return self.dispatch_request(*args, **kwargs)
  File "/home/lefteris/.virtualenvs/raiden/lib/python3.6/site-packages/flask_restful/__init__.py", line 595, in dispatch_request
    resp = meth(*args, **kwargs)
  File "/home/lefteris/.virtualenvs/raiden/lib/python3.6/site-packages/webargs/core.py", line 441, in wrapper
    return func(*args, **kwargs)
  File "/home/lefteris/ew/raiden/raiden/api/v1/resources.py", line 185, in put
    joinable_funds_target=joinable_funds_target,
  File "/home/lefteris/ew/raiden/raiden/api/rest.py", line 470, in connect
    joinable_funds_target,
  File "/home/lefteris/ew/raiden/raiden/api/python.py", line 163, in token_network_connect
    joinable_funds_target=joinable_funds_target,
  File "/home/lefteris/ew/raiden/raiden/connection_manager.py", line 108, in connect
    raise ValueError('connecting needs a positive value for `funds`')
ValueError: connecting needs a positive value for `funds`

Task

Return a proper error through the API instead of the internal server error

[hackaugusto]

@pcppcp does #1959 also fix this?

[LefterisJP]

@hackaugusto no it does not. I am on latest master and just got it. Fix incoming in a few secs.