Profiling servlet ensure output directory exists and is writable

When the profiling servlet runs async-profiler when the output path (/tmp/prof-output) doesn't exist or isn't writable by the hbase process, it will sit in a refresh loop indefinitely. The servlet should check that the output path is writable, or even make an attempt to create the output directory, and inform the user of the situation, rather than attempting and silently failing to generate the flame graph.