Return TOX_ERR_CONFERENCE_SEND_MESSAGE_NO_CONNECTION if we are not connected to any peers

[zugz]

Closes #1269.

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This change is 

[codecov]

Codecov Report

Merging #1271 into master will decrease coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1271     +/-   ##
========================================
- Coverage    83.1%     83%   -0.1%     
========================================
  Files          82      82             
  Lines       14685   14685             
========================================
- Hits        12204   12193     -11     
- Misses       2481    2492     +11

Impacted Files	Coverage Δ
toxcore/group.c	76.2% <100%> (ø)	⬆️
toxav/msi.c	66.2% <0%> (-0.6%)	⬇️
toxcore/net_crypto.c	93.2% <0%> (-0.5%)	⬇️
toxav/toxav.c	68.9% <0%> (-0.4%)	⬇️
toxcore/friend_connection.c	93.1% <0%> (-0.3%)	⬇️
toxcore/DHT.c	76.7% <0%> (-0.1%)	⬇️

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