server: refactor ldap_handler

Split it into several files, move them into the domain folder, introduce
`LdapError` for better control flow.

server/src/domain/ldap/error.rs

@@ -0,0 +1,17 @@
+use ldap3_proto::LdapResultCode;
+
+#[derive(Debug, PartialEq)]
+pub struct LdapError {
+    pub code: LdapResultCode,
+    pub message: String,
+}
+
+impl std::fmt::Display for LdapError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(&self.message)
+    }
+}
+
+impl std::error::Error for LdapError {}
+
+pub type LdapResult<T> = std::result::Result<T, LdapError>;