Config: perform basic validation

This just makes sure the parking posture is within the valid range of
motion of the rotator.

src/main.zig

@@ -69,7 +69,7 @@ pub fn main() !u8 {
             };
             defer conf_file.close();
 
-            Config.load(allocator, conf_file.reader()) catch |err| {
+            Config.load(allocator, conf_file.reader(), std.io.getStdErr().writer()) catch |err| {
                 log.err("Could not parse config file '{s}': {s}.", .{ confpath, @errorName(err) });
                 return 1;
             };