Explicitly return error for empty input files
Explicitly return error for empty input files

Often profile handlers return no data, and pprof currently
returns "profile format unrecognized", which confuses users
and sends them on a wild goose chase.

Print a more explicit "empty input file" error message.

google/pprof,google/pprof,rauls5382/pprof,rauls5382/pprof,google/pprof,google/pprof