Feature/improve user experience

[rhoriguchi]

• Add else block for invalid input
• Add java version validation since it currently does not run on Java 17

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@83d3df5). Click here to learn what that means. The diff coverage is 44.4%.

@@           Coverage Diff            @@
##             master      #9   +/-   ##
========================================
  Coverage          ?   83.2%           
  Complexity        ?      43           
========================================
  Files             ?       5           
  Lines             ?     143           
  Branches          ?      20           
========================================
  Hits              ?     119           
  Misses            ?      12           
  Partials          ?      12           

Impacted Files	Coverage Δ
...n/java/org/contextmapper/cli/ContextMapperCLI.java	68.0% <37.5%> (ø)
...rg/contextmapper/cli/commands/GenerateCommand.java	88.2% <100.0%> (ø)

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