libhdfs++: Improve C API error reporting

The thread local string used for hdfsGetLastError wasn't reset between calls so it could give stale results in confusing ways. Now it gets reset with a placeholder that says that <functionname> hasn't set an error string.

Also fixed indentation that wasn't consistent + marked which functions are used by the hdfs.h API and hdfs_ext.h API to make it easier to see when changes could break compatibility. Included some minor cleanup for the common case catch blocks.