don't treat setDF failed as error

[haruue]

setDF() raised this error and made hysteria fully stop working on Windows 7
apernet/hysteria#327

I tested and seems like ignoring this error won't cause other problems.

And this error handle itself is really weird:

• IPv4 setDF failed => ok
• IPv6 setDF failed => ok
• IPv4 setDF failed && IPv6 setDF failed => raise err (why?)

[codecov-commenter]

Codecov Report

Merging #2 (48248ae) into v0.27.0-mod (ed2a645) will not change coverage.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##           v0.27.0-mod       #2   +/-   ##
============================================
  Coverage        85.35%   85.35%           
============================================
  Files              138      138           
  Lines             9918     9918           
============================================
  Hits              8465     8465           
  Misses            1072     1072           
  Partials           381      381           

Impacted Files	Coverage Δ
sys_conn_df_linux.go	52.94% <0.00%> (ø)
sys_conn_df_windows.go	52.94% <0.00%> (ø)

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