Handle error case when no GPU is found

[webmaster128]

Heyho – thanks for doing this.

After compiling on a server with no GPU I get

$ RUST_BACKTRACE=1 nano-vanity  --gpu .ababab
Estimated attempts needed: 1073741824
thread 'main' panicked at 'Platform::list: Error retrieving platform list: Unable to get platform id list after 10 seconds of waiting.', libcore/result.rs:1009:5
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:477
   5: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:391
   6: rust_begin_unwind
             at libstd/panicking.rs:326
   7: core::panicking::panic_fmt
             at libcore/panicking.rs:77
   8: core::result::unwrap_failed
   9: ocl::standard::platform::Platform::list
  10: nano_vanity::gpu::Gpu::new
  11: nano_vanity::main
  12: std::rt::lang_start::{{closure}}
  13: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  14: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:103
  15: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  16: main
  17: __libc_start_main
  18: _start

It would be nice to show an error message if no GPU was available.