db/DatabaseError: add exception class DatabaseError

src/command/CommandError.cxx

@@ -179,6 +179,10 @@ ToAck(std::exception_ptr ep)
 		return pe.GetCode();
 	} catch (const PlaylistError &pe) {
 		return ToAck(pe.GetCode());
+#ifdef ENABLE_DATABASE
+	} catch (const DatabaseError &de) {
+		return ToAck(de.GetCode());
+#endif
 	} catch (const std::system_error &e) {
 		return ACK_ERROR_SYSTEM;
 #if defined(__GLIBCXX__) && __GLIBCXX__ < 20151204

src/db/DatabaseError.hxx

@@ -20,6 +20,8 @@
 #ifndef MPD_DB_ERROR_HXX
 #define MPD_DB_ERROR_HXX
 
+#include <stdexcept>
+
 class Domain;
 
 enum class DatabaseErrorCode {
@@ -34,6 +36,18 @@ enum class DatabaseErrorCode {
 	CONFLICT,
 };
 
+class DatabaseError final : public std::runtime_error {
+	DatabaseErrorCode code;
+
+public:
+	DatabaseError(DatabaseErrorCode _code, const char *msg)
+		:std::runtime_error(msg), code(_code) {}
+
+	DatabaseErrorCode GetCode() const {
+		return code;
+	}
+};
+
 extern const Domain db_domain;
 
 #endif