Refactor Storage components

[bryanyee33]

Move MainApp.java::initModelManager() to StorageManager.java::readInitialAddressBook() to increase cohesion.
Move saveAddressBook exception handling to JsonAddressBookStorage.java.
Add StorageException.java.

Closes #6

[codecov]

Codecov Report

Attention: Patch coverage is 39.28571% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 74.56%. Comparing base (31ebb75) to head (5b9a802).

Files	Patch %	Lines
.../seedu/address/storage/JsonAddressBookStorage.java	50.00%	4 Missing ⚠️
...ain/java/seedu/address/storage/StorageManager.java	60.00%	3 Missing and 1 partial ⚠️
...u/address/storage/exceptions/StorageException.java	0.00%	4 Missing ⚠️
src/main/java/seedu/address/MainApp.java	0.00%	3 Missing ⚠️
src/main/java/seedu/address/ui/CommandBox.java	0.00%	1 Missing ⚠️
src/main/java/seedu/address/ui/MainWindow.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #55      +/-   ##
============================================
+ Coverage     73.75%   74.56%   +0.80%     
- Complexity      392      397       +5     
============================================
  Files            69       70       +1     
  Lines          1269     1270       +1     
  Branches        105      104       -1     
============================================
+ Hits            936      947      +11     
+ Misses          303      292      -11     
- Partials         30       31       +1     

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[aureliony]

LGTM

[yisiox]

LGTM

[aureliony]

LGTM