when file from mysql_read_default_file is missing there is no error for a missing file

[duronrulez]

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it would be great if the mysql_read_default_file option actually checks if there is a file existing in that location, and throw the appropriate error.

Its a dumb mistake not to check yourself, but i've lost about 10-15 minutes until i figured out that the file is missing as the error was not very helpful (exerpt from pt-online-schema-change tool):

DBI connect(';mysql_read_default_file=/xxx/.my.cnf;host=xxx;mysql_read_default_group=client','',...) failed: Access denied for user 'xxx'@'xxx' (using password: NO) at /usr/bin/pt-show-grants line 1367.