Cope with race creating band

[sourcefrog]

ERROR: File exists (os error 17)
stack backtrace:
   0:        0x100e351be - backtrace::backtrace::trace::h1b789d8e1542dad0
   1:        0x100e358fc - backtrace::capture::Backtrace::new::h3d062d22ca4dc5a6
   2:        0x100e34d36 - error_chain::make_backtrace::h48f950ecdd86ad84
   3:        0x100e34ded - _$LT$error_chain..State$u20$as$u20$core..default..Default$GT$::default::he14c64e0c2fc5de7
   4:        0x100e0a3bc - conserve::band::Band::create::h1d318051cfc1c04f
   5:        0x100e07ca4 - conserve::archive::Archive::create_band::h342b7c2956161944
   6:        0x100e088ba - conserve::backup::backup::hca5a5659f78774db
   7:        0x100d9f81a - conserve::backup::h055dd3b66439ec1d
   8:        0x100d9edd7 - conserve::main::h53c12c360c8dc5eb
   9:        0x100e8c38a - __rust_maybe_catch_panic
  10:        0x100e8b906 - std::rt::lang_start::ha9be7b379cf1665e