Add input validations for `open`

[salty-flower]

...with respect to #193. I made the parser throw a ParseException when no social media platform specified, instead of testing if there's an only platform for the person (as desired by the tester).

[codecov[bot]]

Codecov Report

Merging #216 (621c11a) into master (15723e8) will increase coverage by 0.36%. The diff coverage is 33.33%.

@@             Coverage Diff              @@
##             master     #216      +/-   ##
============================================
+ Coverage     65.59%   65.96%   +0.36%     
- Complexity      672      676       +4     
============================================
  Files            99       99              
  Lines          2404     2412       +8     
  Branches        346      349       +3     
============================================
+ Hits           1577     1591      +14     
+ Misses          659      648      -11     
- Partials        168      173       +5     

Impacted Files	Coverage Δ
...va/seedu/connectus/logic/commands/OpenCommand.java	45.45% <0.00%> (+31.16%)	:arrow_up:
...eedu/connectus/logic/parser/OpenCommandParser.java	66.66% <60.00%> (+66.66%)	:arrow_up:

... and 1 file with indirect coverage changes

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