Fixed bug on Windows that treats MSVC banner as error

Author: Pebaz

nimporter.py

@@ -172,7 +172,10 @@ def compile(cls, module_path, release_mode=False):
         if NIM_COMPILE_ERROR in err:
             raise NimCompilerException(err)
         elif err:
-            raise Exception(err)
+            # NOTE(pebaz): On Windows, Nim spits out the MSVC banner to stderr,
+            # causing `err` to not be None. If it built, ignore the error.
+            if not build_artifact.exists():
+                raise Exception(err)
         elif NIM_COMPILE_ERROR in out:
             raise NimCompilerException(out)