refactor errorhandler for deduplication of logic

Mostly just move a comment around back to where it's more relevant, and
handle MESON_FORCE_TRACEBACK once instead of twice.

mesonbuild/mesonmain.py

@@ -38,10 +38,18 @@ def errorhandler(e, command):
         if os.environ.get('MESON_FORCE_BACKTRACE'):
             raise e
         return 1
-    elif isinstance(e, OSError):
+    else:
+        # We assume many types of traceback are Meson logic bugs, but most
+        # particularly anything coming from the interpreter during `setup`.
+        # Some things definitely aren't:
+        # - PermissionError is always a problem in the user environment
+        # - runpython doesn't run Meson's own code, even though it is
+        #   dispatched by our run()
         if os.environ.get('MESON_FORCE_BACKTRACE'):
             raise e
         traceback.print_exc()
+
+        if isinstance(e, OSError):
             error_msg = os.linesep.join([
                 "Unhandled python exception",
                 f"{e.strerror} - {e.args}",
@@ -49,17 +57,7 @@ def errorhandler(e, command):
 
             mlog.exception(error_msg)
             return e.errno
-
         else: # Exception
-        if os.environ.get('MESON_FORCE_BACKTRACE'):
-            raise e
-        traceback.print_exc()
-        # We assume many types of traceback are Meson logic bugs, but most
-        # particularly anything coming from the interpreter during `setup`.
-        # Some things definitely aren't:
-        # - PermissionError is always a problem in the user environment
-        # - runpython doesn't run Meson's own code, even though it is
-        #   dispatched by our run()
             if command != 'runpython':
                 msg = 'Unhandled python exception'
                 if all(getattr(e, a, None) is not None for a in ['file', 'lineno', 'colno']):