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/builtinimport.c b/py/builtinimport.c
index 7ecf091..c96a7d4 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -118,7 +118,12 @@
 
     if (lex == NULL) {
         // we verified the file exists using stat, but lexer could still fail
-        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ImportError, "No module named '%s'", vstr_str(file)));
+        if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
+            nlr_raise(mp_obj_new_exception_msg(&mp_type_ImportError, "module not found"));
+        } else {
+            nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ImportError,
+                "no module named '%s'", vstr_str(file)));
+        }
     }
 
     #if MICROPY_PY___FILE__
@@ -277,7 +282,12 @@
                 {
                 #endif
                     // couldn't find the file, so fail
-                    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ImportError, "No module named '%s'", qstr_str(mod_name)));
+                    if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
+                        nlr_raise(mp_obj_new_exception_msg(&mp_type_ImportError, "module not found"));
+                    } else {
+                        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ImportError,
+                            "no module named '%s'", qstr_str(mod_name)));
+                    }
                 }
             } else {
                 // found the file, so get the module