Update editloan command to correctly show error message for invalid values

[kjw142857]

Fixes #115.

• When the editloan command is entered with an invalid loan value (e.g. -1), an error message is now correctly show similar to linkloan.
• The parseLoan command has also been greatly simplified due to duplication of code with parseValue and parseDate.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 47.06%. Comparing base (e66deea) to head (2ccde5c).
Report is 2 commits behind head on master.

Files	Patch %	Lines
...in/java/seedu/address/logic/parser/ParserUtil.java	0.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #119      +/-   ##
============================================
+ Coverage     46.90%   47.06%   +0.16%     
  Complexity      433      433              
============================================
  Files            99       99              
  Lines          2275     2267       -8     
  Branches        227      227              
============================================
  Hits           1067     1067              
+ Misses         1163     1155       -8     
  Partials         45       45              

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[Joseph31416]

lgtm!