error happened but not returned to the client

[tiancaiamao]

Enhancement

By reproduce #48411 and after reading the code, I found it might not be the only case that
error happened but not returned to the client.

defer terror.Call(rs.Close())
for {
    data = rs.Next()
    writePacket(data)
}
writeEOF()

terror.Call ignores error, so the error is not returned to the client.

But NOTE, even we to write it as this, the code is still wrong:

defer func() {
    err = rs.Close()
}()
for {
    data = rs.Next()
    writePacket(data)
}
writeEOF()

The problem is that after we writeEOF(), we finish this mysql response telling the client that all the data is done.
So even we find an error later (for example, in defer rs.Close()), there is no way to tell the mysql client any more!

The correct order should be:

for {
    data = rs.Next()
    writePacket(data)
}
if err = rs.Close(); err != nil {
    writeErr()
} else {
    writeEOF()
}