Add propagate=False property to swallow exceptions after on_error

[gabrtv]

When using on_error we've found it useful to swallow exceptions since the FSM module is already handling the state transition. This PR adds an optional propagate property to control this. Behavior defaults to propagate=True matching current behavior.

Note: a test was included but it's unclear how to run the suite, so it might not be passing. Some docs on how to run the tests would be much appreciated.

[kmmbvnr]

Tests could be run with tox command.

Please fix it.

[kmmbvnr]

Also, README need to be updated with new option

[kmmbvnr]

Btw, could we think about the way to restrict set of exceptions?

So, may be we could accept with same option not only Boolean but a list of exception that going to be hidden

not_propagate=[NotFound, DatabaseError] (probably with some better option name)

[kmmbvnr]

Shame on you ) Deis could not be considered 1.0 with unfinished work.

So, my proposed solution is to have new enshroud paramenter, default false.

enshroud=[list of intercepted exception] or enshroud=true to intercept them all

[kmmbvnr]

I'm not sure that such functionality really required. But if it is true, i think it should be implemented in more explicit way like #f56b0ce

Ping here, if someone have a reasons to wish this commit merged to mainline.

[mmoravcik]

@kmmbvnr @gabrtv hi, so when exception happens, do we still need to catch it the code?

try:
    order.submit()
except SomeException:
    pass

In the above example, Django FSM will change the status to on_error, right? Catching the exception might look pointless, I guess that was the reason for propagate=True