Provide status callbacks.

[petehouston]

This includes following updates:

• Callbacks for AbstractWorker on success (::onSucceeded()) and on failure (::onFailed()).
• Query for error if there is, so the implemented workers, inherited from AbstractWorker, can catch error exception to handle.

[codecov]

Codecov Report

Merging #39 into master will decrease coverage by 0.65%.
The diff coverage is 77.77%.

@@             Coverage Diff              @@
##             master      #39      +/-   ##
============================================
- Coverage     94.99%   94.33%   -0.66%     
- Complexity      132      137       +5     
============================================
  Files            18       18              
  Lines           539      547       +8     
============================================
+ Hits            512      516       +4     
- Misses           27       31       +4

Impacted Files	Coverage Δ	Complexity Δ
Service/Worker/AbstractWorker.php	80% <77.77%> (-20%)	10 <4> (+5)

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[trandangtri]

Thanks for your PR, but please take a look at my feedbacks :)