Add informative error

[curzolapierre]

• Add a changelog entry in the section "To Be Released" of CHANGELOG.md

Now errors are printed like:

scalingo --app sample-go-martini scale M:0 Test:0
 !     An error occurred:
       * M → containers type not found
       * Test → containers type not found
       
       Your available container types are: 'web', 'other'.
       
       Example of usage:
       scalingo --app my-app scale web:2 worker:1
       scalingo --app my-app scale web:1:XL
       scalingo --app my-app scale web:+1 worker:-1
       
       Use 'scalingo scale --help' for more information

Related to https://github.com/Scalingo/api/pull/1799
Related to https://github.com/Scalingo/one-stop-shop/issues/191
Related to OPSB-49

[EtienneM]

Suggested change

	reqestFailedError, _ := errors.ErrgoRoot(err).(*http.RequestFailedError)
	reqestFailedError, ok := errors.ErrgoRoot(err).(*http.RequestFailedError)

Shouldn't you test the ok value? What happens if the error is not of type http.RequestFailedError? 🤔

[EtienneM]

Suggested change

	// In case of unprocessable, format and return an clear error
	// In case of unprocessable, format and return a clear error

[EtienneM]

In case of error formatting the error message, I wouldn't return the error. I think it would be best to:

• Log the error
• Display the error message contained in requestFailedError. This error message is already a good error message. If we fail to enrich it with more informative information, we don't want to lose the source of the error message. Don't you think?

[Soulou]

Related to OPSB-49