Append filename to OOM message when dump_on_limit is triggered, and handle dump failures (#65)

Memprof dumps the active profile when in dump_on_limit mode and the
memory_limit is reached.

Here we expose the name of the dump file in the OOM message:

    Allowed memory size of 15728640 bytes exhausted (tried to allocate %d bytes) (memprof dumped to ...)

In case the dump failed, we expose this fact as well:

    Allowed memory size of 15728640 bytes exhausted (tried to allocate %d bytes) (memprof failed dumping to ..., please check file permissions or disk capacity)

Author: arnaud-lb