Display relevant error message for tags with spaces

[ruoyann]

Resolves #170 Can't seem to change the validation regex in the tag class, it throws some error that can't parse the json files

If anyone is interested, I think the validation regex for alphanum with spaces is ^[a-zA-Z0-9_ ]*$ https://stackoverflow.com/questions/15472764/regular-expression-to-allow-spaces-between-words

[codecov-commenter]

Codecov Report

Merging #187 (0020922) into master (3568d61) will decrease coverage by 0.04%. The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #187      +/-   ##
============================================
- Coverage     62.68%   62.64%   -0.05%     
  Complexity      670      670              
============================================
  Files            94       94              
  Lines          2573     2575       +2     
  Branches        332      333       +1     
============================================
  Hits           1613     1613              
- Misses          863      864       +1     
- Partials         97       98       +1     

Impacted Files	Coverage Δ
...in/java/seedu/address/logic/parser/ParserUtil.java	82.41% <0.00%> (-1.86%)	:arrow_down:
src/main/java/seedu/address/model/tag/Tag.java	100.00% <ø> (ø)

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