Bug#18908: ERROR 1406 (22001): Data too long for column :: using utf8

Problem: Too confusing error message when cannot convert
between string and column character sets on INSERT and UPDATE.
Fix: producing a better error message, instead of "Data too long"
in such cases
Additional changes: Adding "DROP TABLE IF EXISTS" into several
tests to be safe against failures in previous tests.

mysql-test/include/strict_autoinc.inc

@@ -2,6 +2,10 @@
 # Test for strict-mode autoincrement
 #
 
+--disable_warnings
+drop table if exists t1;
+--enable_warnings
+
 set @org_mode=@@sql_mode;
 eval create table t1
 (