Merge branch 'MPL_medium_property_error_msg' into 'master'

<MPL> Medium throws an error if a property is not defined in the input file

See merge request ogs/ogs!3337

MaterialLib/MPL/Medium.cpp

@@ -44,6 +44,12 @@ Phase const& Medium::phase(std::string const& name) const
 
 Property const& Medium::property(PropertyType const& p) const
 {
+    Property const* const property = properties_[p].get();
+    if (property == nullptr)
+    {
+        OGS_FATAL("Trying to access undefined property '{:s}' of {:s}",
+                  property_enum_to_string[p], description());
+    }
     return *properties_[p];
 }