Fix/extract integrity

[ErwanRaulo]

command allows the following spec arguments:

• undefined: leading to an error on string.match fn
• express: leading to a null version after spec parsing and thus to a 404 from registry API
• something invalid to parse: that just throw an error

to do:

• log user friendly i18n error message about arguments
  example:

• handle arguments unexpected values
• add more unit tests

what could be better ?:

• migrate commands to typescript strict mode

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.46%. Comparing base (28ee6a9) to head (24b54f5).
⚠️ Report is 301 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (28ee6a9) and HEAD (24b54f5). Click for more details.

HEAD has 3 uploads less than BASE

Flag	BASE (28ee6a9)	HEAD (24b54f5)
	6	3

Additional details and impacted files

@@             Coverage Diff              @@
##            master     #647       +/-   ##
============================================
- Coverage   100.00%   59.46%   -40.54%     
============================================
  Files           15       46       +31     
  Lines          649     3498     +2849     
  Branches         0       15       +15     
============================================
+ Hits           649     2080     +1431     
- Misses           0     1418     +1418     

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