resources: Output error message in downloader.py

This allows for the actual error message to be output in addition
to the output gem5 has on ValueErrors and ImportErrors.

Change-Id: Ic52f5646aa41dbf7c217ab27d142c0a18fa24c55
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/71859
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/python/gem5/resources/downloader.py

@@ -157,12 +157,14 @@ def _download(url: str, download_to: str, max_attempts: int = 6) -> None:
                 raise e
         except ValueError as e:
             raise Exception(
+                f"ValueError: {e}\n"
                 "Environment variable GEM5_USE_PROXY is set to "
                 f"'{use_proxy}'. The expected form is "
                 "<host>:<port>'."
             )
         except ImportError as e:
             raise Exception(
+                f"ImportError: {e}\n"
                 "An import error has occurred. This is likely due "
                 "the Python SOCKS client module not being "
                 "installed. It can be installed with "