py: Use shorter, static error msgs when ERROR_REPORTING_TERSE enabled.

Going from MICROPY_ERROR_REPORTING_NORMAL to
MICROPY_ERROR_REPORTING_TERSE now saves 2020 bytes ROM for ARM Thumb2,
and 2200 bytes ROM for 32-bit x86.

This is about a 2.5% code size reduction for bare-arm.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 02c6674..0c214bb 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -238,7 +238,7 @@
 #define MICROPY_ENABLE_DOC_STRING (0)
 #endif
 
-// Exception messages are short static strings (TODO)
+// Exception messages are short static strings
 #define MICROPY_ERROR_REPORTING_TERSE    (1)
 // Exception messages provide basic error details
 #define MICROPY_ERROR_REPORTING_NORMAL   (2)