vtls_openssl: improve client certificate load failure error messages

lib/vtls/openssl.c

@@ -403,7 +403,10 @@ int cert_stuff(struct connectdata *conn,
       /* SSL_CTX_use_certificate_chain_file() only works on PEM files */
       if(SSL_CTX_use_certificate_chain_file(ctx,
                                             cert_file) != 1) {
-        failf(data, SSL_CLIENT_CERT_ERR);
+        failf(data,
+              "could not load PEM client certificate, OpenSSL error %s, "
+              "(no key found, wrong pass phrase, or wrong file format?)",
+              ERR_error_string(ERR_get_error(), NULL) );
         return 0;
       }
       break;
@@ -415,7 +418,10 @@ int cert_stuff(struct connectdata *conn,
       if(SSL_CTX_use_certificate_file(ctx,
                                       cert_file,
                                       file_type) != 1) {
-        failf(data, SSL_CLIENT_CERT_ERR);
+        failf(data,
+              "could not load ASN1 client certificate, OpenSSL error %s, "
+              "(no key found, wrong pass phrase, or wrong file format?)",
+              ERR_error_string(ERR_get_error(), NULL) );
         return 0;
       }
       break;