define exception hierarchy

allow is_running to return values x>=0

src/nvda_voicemeeter/errors.py

@@ -0,0 +1,11 @@
+class NVDAVMError(Exception):
+    """Base NVDAVM error class"""
+
+
+class NVDAVMCAPIError(NVDAVMError):
+    """Exception raised when the NVDA C-API returns an error code"""
+
+    def __init__(self, fn_name, code):
+        self.fn_name = fn_name
+        self.code = code
+        super().__init__(f"{self.fn_name} returned {self.code}")