Avoid redundant str cast (#867)

- see https://docs.python.org/3/library/string.html#format-specification-mini-language

pynetdicom/service_class.py

@@ -289,7 +289,7 @@ def _c_find_scp(self, req: C_FIND, context: "PresentationContext") -> None:
  LOGGER.error(
  "\nTraceback (most recent call last):\n"
  + "".join(traceback.format_tb(exc[2]))
- + f"{exc[0].__name__}: {str(exc[1])}" # type: ignore
+ + f"{exc[0].__name__}: {exc[1]}" # type: ignore
  )
  rsp_status = 0xC311
  dataset = None
@@ -1707,7 +1707,7 @@ def _add_failed_instance(ds: Dataset) -> None:
  LOGGER.error(
  "\nTraceback (most recent call last):\n"
  + "".join(traceback.format_tb(exc[2]))
- + f"{exc[0].__name__}: {str(exc[1])}" # type: ignore
+ + f"{exc[0].__name__}: {exc[1]}" # type: ignore
  )
  rsp_status = 0xC411
  dataset = None
@@ -2131,7 +2131,7 @@ def _add_failed_instance(ds: Dataset) -> None:
  LOGGER.error(
  "\nTraceback (most recent call last):\n"
  + "".join(traceback.format_tb(exc[2]))
- + f"{exc[0].__name__}: {str(exc[1])}" # type: ignore
+ + f"{exc[0].__name__}: {exc[1]}" # type: ignore
  )
  rsp_status = 0xC511
  dataset = None