Addressed following TODO and added error reporting functionality: 

// TODO: detect errors by checking the metricsValues and ticChartUnit
fields.

ReportReader.java: 
Added condition to check missing values from metricsValues. 
Added condition to check value of maxIntensity from ticChartUnit.
Uncommented code for checking data files availability.
Set error check for missing data files. 
Removed code for saving error messages.
Added useful constants.

ViewerFrame.java: 
Added JLabel component to show Missing data label in case of errors in
the report.
Using getReportIndex instead of getReportNum in actionPerformed(...)
method.

ReportUnit.java:
Reverted to previous version of compare(...).
Added TODO: Analyze new version of compare(...) for correct sorting
mechanism.

Constants.java:
Added some useful constants.