Catch warpx not being initialized in library loader (#5567)

Follows up on, #5412, using
@ax3l's
[suggestion](#5412 (review))
to catch warpx not being initialized when accessing libwarpx.warpx

Author: archermarx

Python/pywarpx/_libwarpx.py

@@ -40,9 +40,14 @@ def __getattr__(self, attribute):
             # Once loaded, it gets added to the dictionary so this code won't be called again.
             self.load_library()
             return self.__dict__[attribute]
+        elif attribute == "warpx":
+            # A `warpx` attribute has not yet been assigned, so `initialize_warpx` has not been called.
+            raise AttributeError(
+                "Trying to access libwarpx.warpx before initialize_warpx has been called!"
+            )
         else:
             # For any other attribute, call the built-in routine - this should always
-            # return an AttributeException.
+            # return an AttributeError.
             return self.__getattribute__(attribute)
 
     def _get_package_root(self):
@@ -143,8 +148,6 @@ def initialize(self, argv=None, mpi_comm=None):
         self.libwarpx_so.execute_python_callback("afterinit")
         self.libwarpx_so.execute_python_callback("particleloader")
 
-        # self.libwarpx_so.warpx_init()
-
         self.initialized = True
 
     def finalize(self, finalize_mpi=1):

Python/pywarpx/particle_containers.py

@@ -35,7 +35,7 @@ def particle_container(self):
                     self.name
                 )
             except AttributeError as e:
-                msg = "This is likely caused by attempting to access a ParticleContainerWrapper before initialize_warpx has been called"
+                msg = "You must initialize WarpX before accessing a ParticleContainerWrapper's particle_container."
                 raise AttributeError(msg) from e
 
         return self._particle_container
@@ -777,7 +777,7 @@ def particle_buffer(self):
             try:
                 self._particle_buffer = libwarpx.warpx.get_particle_boundary_buffer()
             except AttributeError as e:
-                msg = "This is likely caused by attempting to access a ParticleBoundaryBufferWrapper before initialize_warpx has been called"
+                msg = "You must initialize WarpX before accessing a ParticleBoundaryBufferWrapper's particle_buffer."
                 raise AttributeError(msg) from e
 
         return self._particle_buffer