C++ client error memory should be bounded

The Kudu client can allocate unbounded amounts of memory for error messages today, which is a potential problem for processes using the client. The API to get errors does have a way to indicate the errors overflowed the error buffer, but this isn't set yet.

Given that error messages can vary significantly in size (as they include a var length string which will soon also include tracing output) and the row, the client should have a way to indicate the max buffer memory size rather than just some number of rows.