explicitly check if the config is readable before continuing

Previously, loading a config as a python module reported this failure
to exist check for us rather nicely and so we didn't need to worry about
doing it ourselves. Now, we need to do it ourselves since the
Py_RunAnyFile* functions do not check for existence manually and will
fail strongly if they are given a nullptr.

src/Framework/ConfigurePython.cxx

@@ -275,6 +275,17 @@ ConfigurePython::ConfigurePython(const std::string& pythonScript, char* args[],
   // assumes that nargs >= 0
   //  this is true always because we error out if no python script has been
   //  found
+
+  // load a handle to the config file into memory (and check that it exists)
+  FILE* config_file{fopen(pythonScript.c_str(), "r")};
+  if (config_file == NULL) {
+    EXCEPTION_RAISE(
+        "ConfigDNE",
+        "Passed config script '"+pythonScript+"' is not accessible.\n"
+        "    Did you make a typo in the path to the script?\n"
+        "    Are you referencing a directory that is not mounted to the container?"
+        );
+  }
 
   std::string cmd = pythonScript;
   if (pythonScript.rfind("/") != std::string::npos) {
@@ -302,8 +313,6 @@ ConfigurePython::ConfigurePython(const std::string& pythonScript, char* args[],
   // the name of the python script
   PySys_SetArgvEx(nargs + 1, targs, 1);
 
-  // load a handle to the config file into memory
-  FILE* config_file{fopen(pythonScript.c_str(), "r")};
   // run the file as a python script
   if (PyRun_AnyFile(config_file, pythonScript.c_str()) != 0) {
     PyErr_Print();