Ensure verify error is set when X509_verify_cert() fails

Set ctx->error = X509_V_ERR_OUT_OF_MEM when verificaiton cannot
continue due to malloc failure.  Similarly for issuer lookup failures
and caller errors (bad parameters or invalid state).

Also, when X509_verify_cert() returns <= 0 make sure that the
verification status does not remain X509_V_OK, as a last resort set
it it to X509_V_ERR_UNSPECIFIED, just in case some code path returns
an error without setting an appropriate value of ctx->error.

Add new and some missing error codes to X509 error -> SSL alert switch.

Reviewed-by: Tim Hudson <tjh@openssl.org>