wordnik.py: Fix various bugs, clean up plugin

[linuxdaemon]

• Handle unknown data sources
• Handle API error response
• Clean up command responses to be more consistent
• Add tests

[codecov[bot]]

Codecov Report

Merging #546 into gonzobot will increase coverage by 1.19%. The diff coverage is 100%.

@@             Coverage Diff              @@
##           gonzobot     #546      +/-   ##
============================================
+ Coverage     48.03%   49.23%   +1.19%     
============================================
  Files           226      224       -2     
  Lines         13369    13558     +189     
============================================
+ Hits           6422     6675     +253     
+ Misses         6947     6883      -64

Impacted Files	Coverage Δ
tests/plugin_tests/test_wordnik.py	100% <100%> (ø)
plugins/wordnik.py	100% <100%> (+85.71%)	:arrow_up:
plugins/twitter.py	17.35% <0%> (-22.02%)	:arrow_down:
plugins/youtube.py	17.74% <0%> (-18.2%)	:arrow_down:
cloudbot/util/http.py	64.28% <0%> (-2.76%)	:arrow_down:
plugins/tell.py	44.67% <0%> (-0.28%)	:arrow_down:
plugins/link_announcer.py	100% <0%> (ø)	:arrow_up:
tests/plugin_tests/test_utility.py	100% <0%> (ø)	:arrow_up:
plugins/myfitnesspal.py	17.85% <0%> (ø)	:arrow_up:
plugins/xkcd.py	62.85% <0%> (ø)	:arrow_up:
... and 25 more

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