Created DatePickerBtn and Started Using it in ProgressCreationScreen

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

lib/models/app_exceptions.dart

@@ -5,3 +5,14 @@ class TimeProgressInvalidNameException implements Exception {
 
   String errMsg() => "The name of a TimeProgress can't be: $invalidName";
 }
+
+class TimeProgressStartTimeIsNotBeforeEndTimeException implements Exception {
+  final startTime;
+  final endTime;
+
+  TimeProgressStartTimeIsNotBeforeEndTimeException(
+      this.startTime, this.endTime);
+
+  String errMsg() =>
+      "The Start Time has to be before the end time. Therefore these values are invalid: Start Time: $startTime EndTime: $endTime";
+}