Feature/Fix-Invalid-TimeProgress-Name (#3)

Throw Exception when creating time progress with empty or null name.
Handle Exception when user creates new time progress or changes name.

Signed-off-by Andreas Fahrecker <AndreasFahrecker@gmail.com>

lib/models/app_exceptions.dart

@@ -0,0 +1,7 @@
+class TimeProgressInvalidNameException implements Exception {
+  final invalidName;
+
+  TimeProgressInvalidNameException(this.invalidName);
+
+  String errMsg() => "The name of a TimeProgress can't be: $invalidName";
+}