GPIO activity returns done := true on error

[emilienthomas]

I'm submitting a ... (check one with "x")

[X] bug report => search github for a similar issue or PR before submitting
[] enhancement request
[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
When the GPIO activity encounters or creates an error, the Eval function returns done := true, breaking the activity coding best practice:

Activity Eval() should return true only when completed successfully & err should be returned when an error occurs

Expected behavior:
When err is not nil, done must be equal to true

Minimal steps to reproduce the problem (not required if feature enhancement):

What is the motivation / use case for changing the behavior?
Coherent behavior with other activities

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):
GPIO activity requires a raspberry

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.5.6, 0.5.7RC1

Additional information you deem important (e.g. issue happens only occasionally):