to remove

Sql_condition*
raise_condition(const Sql_condition *cond)
{
  Sql_condition *raised= raise_condition(cond->get_sql_errno(),
                                         cond->get_sqlstate(),
                                         cond->get_level(),
                                         *cond,
                                         cond->get_message_text());
  return raised;
}

procedure

Analysis: m_current_row_for_warning is reset to 1 during cleanup phase of
stored procedure. When we perform a copy because some statement of procedure
created warning, this reset value is passed to push_warning().
Hence the output is always 1.
Fix: Add a parameter in relevant functions to pass correct value of
row_number and don't use m_current_row_for_warning directly.

Fixup for MDEV-10075

Analysis: ERROR_INDEX implemented in MDEV-10075 was not intuitively clear.

Fix: changed parser to use ROW_NUMBER instead of ERROR_INDEX. Removed
ERROR_INDEX and ERROR_INDEX_SYM from related files. Changed m_error_index
to m_row_number.

use existing Warning_info::m_current_row_for_warning instead
of a newly introduced counter.

But use m_current_row_for_warning to count rows also in the parser
and during prepare.

Extended the parser for GET DIAGNOSTICS to use ERROR_INDEX to get
warning/error index.
Error information is stored in Sql_condition. So it can be used to
store the index of warning/error too. THD::current_insert_index keeps a
track of count for each row that is processed or going to be inserted in the
table (or first row in case of prepare phase). When an error occurs,
first we need to fetch corrected error index (using correct_error_index())
for an error number. This is needed because in prepare phase, the error
may not be because of rows/values. In such case, correct value of
error_index should be 0. Once correct value if fetched, assign it to
Sql_condition::error_index when the object is created during error/warning.
This error_index variable is returned when ERROR_INDEX is used in
GET DIAGNOSTICS.

The problem is that array binding uses net buffer to read parameters for each
execution while each execiting with RETURNING write in the same buffer.

Solution is to allocate new net buffer to avoid changing buffer we are reading
from.

This allows us to use String::append() without using strlen().

The changes to the ErrConvString class where done by Alexander Barkov

The COM_MULTI did not take off. No connector is using it.
Remove related code from server, and client.
If anything it is a step simplification of already-bloated
dispatch_command(), and related code.