Improve signals dispatching

[yuri-sergiichuk]

This PR fixes #1318.

In the PR I have relaxed the HandlerMap check on missing handlers while it is totally OK to not have a specific handler within a specific class. It's then up to the caller to verify and check how to behave when a handler is missing. In our case, all the respective callers already do check and filter out situations when no handlers are available. The only change one may notice is the error text available via the HandlerFailedUnexpectedly event, instead of the IllegalStateException, we'll see the EventValidationError with the UNSUPPORTED_EVENT code.

[codecov]

Codecov Report

Merging #1343 (4bd38de) into master (29e6531) will increase coverage by 0.07%.
The diff coverage is 92.85%.

@@             Coverage Diff              @@
##             master    #1343      +/-   ##
============================================
+ Coverage     90.98%   91.05%   +0.07%     
+ Complexity     4768     4767       -1     
============================================
  Files           614      614              
  Lines         15139    15135       -4     
  Branches        860      858       -2     
============================================
+ Hits          13774    13781       +7     
+ Misses         1090     1085       -5     
+ Partials        275      269       -6     

[yuri-sergiichuk]

@armiol PTAL.

[armiol]

@yuri-sergiichuk LGTM with a minor comment to address.

[armiol]

Could you please format it like this:

        ImmutableSet<StateClass> result =
                stateHandlers
                        .stream()
                        .filter(StateSubscriberMethod.class::isInstance)
                        .map(StateSubscriberMethod.class::cast)
                        .filter(external ? HandlerMethod::isExternal : HandlerMethod::isDomestic)
                        .map(StateSubscriberMethod::stateType)
                        .map(StateClass::from)
                        .collect(toImmutableSet());