First pass at updating verifier error docs

First pass at updating verifier error docs

X509_verify_cert_error_string() is now thread safe as it no longer returns
a static buffer.  Document X509_V_ERR_UNSPECIFIED.  Stop asserting that the
X509_V_ERR_CERT_CHAIN_TOO_LONG code is unused, the new verifier can set it.
Add commented versions of various missing error codes in the proper spots
and move X509_V_ERR_UNNESTED_RESOURCE where it belongs.

prompted by claudio