Provide sentinel errors in the HTTP API

[sanderpick]

Matching errors against concrete types would be nice vs. fragile error string matching, e.g.,

if err.Error() == "'from' cid was not recursively pinned already" { ...

Or, maybe using more fine grained error codes would be even better. I believe the above error will come with a code: ErrNormal or similar.

[Stebalien]

At the moment, our error codes are mostly concerned with transport level errors:

• ErrNormal - normal error.
• ErrClient - client passed an invalid input.
• ErrImplementation - bug.

We could definitely extend these with additional well-known error codes. That is:

• Errors 0-100: generic errors.
• Errors 100+: other errors.

[sanderpick]

Sounds good, that would be quite nice!