bpo-40246: Report a better error message for invalid string prefixes (GH-19476)

diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 95571a8..eb9159f 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -1574,6 +1574,9 @@
     case E_BADSINGLE:
         msg = "multiple statements found while compiling a single statement";
         break;
+    case E_BADPREFIX:
+        msg = "invalid string prefix";
+        break;
     default:
         fprintf(stderr, "error=%d\n", err->error);
         msg = "unknown parsing error";