Added another XGA error, where a telescope name is not recognised or …

…supported

xga/exceptions.py

@@ -1,5 +1,5 @@
 #  This code is a part of X-ray: Generate and Analyse (XGA), a module designed for the XMM Cluster Survey (XCS).
-#  Last modified by David J Turner (turne540@msu.edu) 22/09/2023, 16:27. Copyright (c) The Contributors
+#  Last modified by David J Turner (turne540@msu.edu) 22/09/2023, 16:37. Copyright (c) The Contributors
 
 
 class HeasoftError(Exception):
@@ -633,3 +633,22 @@ def __str__(self):
         else:
             return 'NoTelescopeDataError has been raised'
 
+
+class InvalidTelescopeError(Exception):
+    def __init__(self, *args):
+        """
+        Raised when the name of a telescope has been passed to a function, but it is not recognised or supported.
+        :param expression:
+        :param message:
+        """
+        if args:
+            self.message = args[0]
+        else:
+            self.message = None
+
+    def __str__(self):
+        if self.message:
+            return '{}'.format(self.message)
+        else:
+            return 'InvalidTelescopeError has been raised'
+