Make plugin import errors more helpful

This addresses an issue where if the user happens to have black installed in
their environment but not the other dependencies when running the protoc
plugin then the resulting import error (No module named 'google') is not very
helpful.

betterproto/tests/inputs/config.py

@@ -11,7 +11,7 @@ tests = {
     "googletypes_service_returns_empty",  # 9
     "casing_message_field_uppercase",  # 11
     "namespace_keywords",  # 70
-    "namespace_builtin_types"  # 53
+    "namespace_builtin_types",  # 53
 }
 
 services = {