consolidate append=true behavior of MySQL.load

[psteinb]

Thanks again for providing and supporting the library.

This is a a suggestion to avoid triggering a warning. If a user calls MySQL.load with append=true, I think she is likely to assume or know that the table under name already exists. Hence calling createtable to me doesn't make too much sense. For this, I suggest to put the entire try-catch clasue in the !append if statement.

[TianChengJiang9911]

We still want to create table at the first time even if append=true. I think maybe it should be like:

try
    createtable(conn, name, sch; quoteidentifiers=quoteidentifiers, debug=debug, createtableclause=append ? "CREATE TABLE IF NOT EXISTS" : "CREATE TABLE", kw...)
catch e

[quinnj]

Thanks for the contribution! However, this change would be breaking - users who call MySQL.load(...; append=true) on a new table would get an error since the table creation is skipped entirely.

I'll implement the suggested CREATE TABLE IF NOT EXISTS approach separately, which achieves the goal of suppressing the warning on subsequent appends without breaking the first-load case.