fix error message when the UDP receive buffer size can't be increased

[marten-seemann]

As reported by @mholt in caddyserver/caddy#3999 (comment).

[codecov]

Codecov Report

Merging #3039 (369442a) into master (0b97ae5) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3039      +/-   ##
==========================================
- Coverage   86.05%   86.01%   -0.04%     
==========================================
  Files         135      135              
  Lines        9472     9801     +329     
==========================================
+ Hits         8151     8430     +279     
- Misses        961     1013      +52     
+ Partials      360      358       -2     

Impacted Files	Coverage Δ
packet_handler_map.go	74.27% <0.00%> (ø)
internal/protocol/version.go	100.00% <0.00%> (ø)
internal/handshake/initial_aead.go	100.00% <0.00%> (ø)
internal/handshake/tls_extension_handler.go	100.00% <0.00%> (ø)
qlog/qlog.go	92.02% <0.00%> (+0.76%)	⬆️
qlog/trace.go	92.00% <0.00%> (+1.52%)	⬆️
internal/handshake/crypto_setup.go	69.96% <0.00%> (+3.38%)	⬆️
http3/server.go	73.49% <0.00%> (+4.43%)	⬆️

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[mholt]

👍