Improve Code Quality of codebase

[B-enguin]

Add assertions at various points of the code.

Bug fixing of messages when loading of Json file fail to match constraints as stated in UG.

Closes #483.

[codecov]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (ff44e93) 84.02% compared to head (ebb775c) 83.54%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #484      +/-   ##
============================================
- Coverage     84.02%   83.54%   -0.48%     
- Complexity      984      986       +2     
============================================
  Files           121      121              
  Lines          2904     2929      +25     
  Branches        342      353      +11     
============================================
+ Hits           2440     2447       +7     
  Misses          397      397              
- Partials         67       85      +18     

Files	Coverage Δ
...s/logic/commands/customer/CustomerListCommand.java	100.00% <100.00%> (ø)
...s/logic/commands/delivery/DeliveryFindCommand.java	85.71% <100.00%> (+0.71%)	⬆️
...ava/seedu/address/storage/JsonAdaptedDelivery.java	100.00% <100.00%> (ø)
.../seedu/address/storage/JsonAddressBookStorage.java	100.00% <100.00%> (ø)
...seedu/address/storage/JsonDeliveryBookStorage.java	77.14% <100.00%> (+1.38%)	⬆️
...s/logic/commands/customer/CustomerViewCommand.java	84.61% <50.00%> (-15.39%)	⬇️
...c/commands/delivery/DeliveryCreateNoteCommand.java	89.74% <0.00%> (-4.86%)	⬇️
...logic/commands/delivery/DeliveryStatusCommand.java	89.47% <0.00%> (-4.98%)	⬇️
.../address/storage/JsonSerializableDeliveryBook.java	78.94% <0.00%> (-21.06%)	⬇️
...ain/java/seedu/address/storage/StorageManager.java	76.92% <0.00%> (-19.86%)	⬇️

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[juliusgambe]

LGTM