Add c.nbrErrors checks

[bengivre]

FIX: issue 71 - Denial of service possible (DoS)

go-smtp$ go test
PASS
ok  	github.com/emersion/go-smtp	0.014s

[codecov]

Codecov Report

Merging #72 into master will decrease coverage by 0.54%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   63.81%   63.27%   -0.55%     
==========================================
  Files           8        8              
  Lines         818      825       +7     
==========================================
  Hits          522      522              
- Misses        217      224       +7     
  Partials       79       79

Impacted Files	Coverage Δ
conn.go	59.94% <0%> (-0.71%)	⬇️
server.go	55.55% <0%> (-1.74%)	⬇️

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[emersion]

Thanks for your PR! Can we extract this into a function, so that changing the message/code/limit/something else doesn't require patching three different locations?

[foxcpp]

@BenoitGEORGELIN, any updates on this?

[emersion]

Superseded by #116.