Report errors when opening config files

The error handling when opening config files was ignoring all errors
except ENOEXIST.  Report other errors, instead of passing nil to
json.NewDecoder and getting:
config file: out/soong/soong.config did not parse correctly: invalid argument

Bug: 73951413
Test: touch out/soong/soong.config && chmod a-r out/soong/soong.config && m
Test: rm out/soong/soong.config && m
Change-Id: I4a609b7f060b760b76ee829b83c0eb405340f58f
diff --git a/android/config.go b/android/config.go
index 1a0b45a..b158e05 100644
--- a/android/config.go
+++ b/android/config.go
@@ -132,12 +132,14 @@
 		if err != nil {
 			return err
 		}
+	} else if err != nil {
+		return fmt.Errorf("config file: could not open %s: %s", filename, err.Error())
 	} else {
 		// Make a decoder for it
 		jsonDecoder := json.NewDecoder(configFileReader)
 		err = jsonDecoder.Decode(configurable)
 		if err != nil {
-			return fmt.Errorf("config file: %s did not parse correctly: "+err.Error(), filename)
+			return fmt.Errorf("config file: %s did not parse correctly: %s", filename, err.Error())
 		}
 	}