Fix delete tag feature

[zekone]

Previously, non existent tags in a contact were accepted in the delete tag command and showed a success message. This behavior has been changed to notifying the user if the tags are non existent. The command will still be executed as usual, with no errors thrown.

[codecov]

Codecov Report

Merging #154 (79113cd) into master (9cedc7b) will increase coverage by 0.03%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master     #154      +/-   ##
============================================
+ Coverage     71.92%   71.96%   +0.03%     
- Complexity      685      686       +1     
============================================
  Files           111      111              
  Lines          2212     2222      +10     
  Branches        250      252       +2     
============================================
+ Hits           1591     1599       +8     
- Misses          528      529       +1     
- Partials         93       94       +1     

Files	Coverage Δ
...seedu/address/logic/commands/DeleteTagCommand.java	78.94% <83.33%> (+0.37%)	⬆️

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[Nixx162]

LGTM