Fix typo in OCSP nonce extension.

Set correct type in ASN1_STRING for
INTEGER and ENUMERATED types.

Make ASN1_INTEGER_get() and ASN1_ENUMERATED_get()
return -1 for invalid type rather than 0 (which is
often valid). -1 may also be valid but this is less
likely.

Load OCSP error strings in ERR_load_crypto_strings().