Issue 137 Fix the response format in the delete and purchase endpoints

[yuhanzz]

1. Added JSON format error handlers and test cases
2. Prevented data validation error caused by deserialize() in update endpoint since we support partial update
3. Added checking for any missing parameter in purchase endpoint
4. Fixed JS code to prevent calls to api/products/ when product id is empty
5. Changed response of purchase endpoint to JSON format

note: the 415 handler can not be covered by test case because of spec-first/connexion#854

[codecov-io]

Codecov Report

Merging #138 (5ca0f6d) into master (7154cc8) will decrease coverage by 0.52%.
The diff coverage is 94.87%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
- Coverage   99.45%   98.93%   -0.53%     
==========================================
  Files           2        2              
  Lines         364      374      +10     
==========================================
+ Hits          362      370       +8     
- Misses          2        4       +2     

Impacted Files	Coverage Δ
service/service.py	98.35% <94.87%> (-0.79%)	⬇️

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