Nicer error

It's okay to assume 3.x for now; there may never be a 4.x protocol and
easy enough to fix if there is.

Author: arp242

conn.go

@@ -1247,7 +1247,7 @@ func (cn *conn) startup(cfg Config) error {
 			newestMinor := r.int32()
 			serverVersion := proto.ProtocolVersion30&0xFFFF0000 | newestMinor
 			if serverVersion < cfg.MinProtocolVersion.proto() {
-				return fmt.Errorf("pq: server does not support minimum protocol version (server newest minor: %d)", newestMinor)
+				return fmt.Errorf("pq: protocol version mismatch: min_protocol_version=%s; server supports up to 3.%d", cfg.MinProtocolVersion, newestMinor)
 			}
 		case proto.ReadyForQuery:
 			cn.processReadyForQuery(r)

connector_test.go

@@ -818,7 +818,7 @@ func TestProtocolVersion(t *testing.T) {
 		{3.2, "3.2", "3.2", key32, ""},
 
 		{3.0, "3.0", "3.2", key30, ""},
-		{3.0, "3.2", "3.2", key30, `server does not support minimum protocol version (server newest minor: 0)`},
+		{3.0, "3.2", "3.2", key30, `pq: protocol version mismatch: min_protocol_version=3.2; server supports up to 3.0`},
 
 		{3.2, "3.9", "3.0", nil, `"3.9" is not supported`},
 		{3.2, "3.0", "3.9", nil, `"3.9" is not supported`},