fix error message when a go string-query fails to parse

generate/testdata/parsing-errors/fragments.go

@@ -0,0 +1,7 @@
+package parsing_errors
+
+var _ = `# @genqlient
+	query myBadQuery(varMissingDollar: String) {
+	  field(arg: $varMissingDollar)
+	}
+`

generate/testdata/parsing-errors/fragments.graphql

@@ -0,0 +1,3 @@
+query myBadQuery(varMissingDollar: String) {
+  field(arg: $varMissingDollar)
+}