WL#9686: Increase the default for max_error_count to 1024

Currently the number of errors, warnings and note messages stored for display by 
SHOW ERRORS and SHOW WARNINGS is truncated at 64.

We have had a community request to increase it to 1024.

There are no static allocations, so will only affect memory consumption for
statements that generate lots of warnings.

The usecase here is: an UPDATE statement that touches 1000s of rows and
many of them give conversion warnings.

It is common for many tools to batch updates, to help reduce replication lag.

External tools like pt-online-schema-change defaults to 1000, and gh-ost
defaults to 100. We cover full error history for both common use cases.

Functional Requirement
======================

Default value of variable 'max_error_count' at server start should be : 1024

The worklog changes the default value of variable :
max_error_count  -   1024       ( new default value )
                       64       ( old default value )

Documentation Link:
https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_max_error_count