Fix exit code on error

[patilpankaj212]

• This PR fixes Terrascan doesn't exit with error on CLI or Parsing errors. #442 (addressed both scan and init commands)
• Removed the code for scan as default command. The function doesn't serve the purpose. Ideally if no commands are specified then help should be printed and if there is a typo in command then cobra prints out useful message with hints.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #458 (c4b9aa7) into master (0f79088) will increase coverage by 0.34%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #458      +/-   ##
==========================================
+ Coverage   74.11%   74.46%   +0.34%     
==========================================
  Files          87       87              
  Lines        2013     2005       -8     
==========================================
+ Hits         1492     1493       +1     
+ Misses        388      379       -9     
  Partials      133      133              

Impacted Files	Coverage Δ
pkg/cli/register.go	6.25% <ø> (+2.25%)	⬆️
pkg/cli/scan.go	77.77% <0.00%> (ø)
pkg/cli/init.go	40.00% <50.00%> (+15.00%)	⬆️