update error response as an object or as a string

[fimbault]

Closes #79

This issue also checks all errors :

• in section 1 : when referring to errors, it is a generic term / no change needed
• same in the security considerations
• elsewhere we aim to have specific errors

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[fimbault]

This doesn't build, will look at that tomorrow

[jricher]

@fimbault : the build errors are due to the IANA section anchors being changed: you have a reference to the (old) IANA-error-response-template which should now be IANA-error-code-template, and you need to update the anchor for IANA-error-response-contents to IANA-error-code-contents. I only see those two but do a text search on those anchors.

[fimbault]

thanks @jricher for the hint, i'll fix that