result set when SQLEXCEPTION is active.

The problem was in a hackish THD::no_warnings_for_error attribute.
When it was set, an error was not written to Warning_info -- only
Diagnostics_area state was changed. That means, Diagnostics_area
might contain error state, which is not present in Warning_info.

The user-visible problem was that in some cases SHOW WARNINGS
returned empty result set (i.e. there were no warnings) while
the previous SQL statement failed. According to the MySQL
protocol errors must be presented in warning list.

The main idea of this patch is to remove THD::no_warnings_for_error.
There were few places where it was used:
  - sql_admin.cc, handling of REPAIR TABLE USE_FRM.
  - sql_show.cc, when calling fill_schema_table_from_frm().
  - sql_show.cc, when calling fill_table().
The fix is to either use internal-error-handlers, or to use
temporary Warning_info storing warnings, which might be ignored.

This patch is needed to fix Bug 11763162 (55843).