openocd: finer-grained error msg if board unknown

When trying to set up tockloader to use openocd, I kept getting
the generic message saying "Could not determine the current board
or arch or openocd board name". Not only did it not mention missing
page_size parameter, but also was too coarse-grained. With splitting
the error message, I managed to debug this, so I believe this could be
beneficial to anyone.

tockloader/openocd.py

@@ -369,14 +369,21 @@ def determine_current_board(self):
         self._configure_from_known_boards()
 
         # Check that we learned what we needed to learn.
-        if (
-            self.board == None
-            or self.arch == None
-            or self.openocd_board == "cortex-m0"
-            or self.page_size == 0
-        ):
+        if self.board is None:
             raise TockLoaderException(
-                "Could not determine the current board or arch or openocd board name"
+                "Could not determine the current board"
+            )
+        if self.arch is None:
+            raise TockLoaderException(
+                "Could not determine the current arch"
+            )
+        if self.openocd_board == "cortex-m0":
+            raise TockLoaderException(
+                "Could not determine the current openocd board name"
+            )
+        if self.page_size == 0:
+            raise TockLoaderException(
+                "Could not determine the current page size"
             )
 
     def run_terminal(self):