fix: Fix error message when a jibri session already exists.

[bgrozev]

No description provided.

[bgrozev]

This changes the error condition, so it will be rendered differently in jitsi-meet. I think that's fine, as it's either a rare race condition or a bug in the client. Any thoughts @saghul (see JID-8726 for more context)

[codecov]

Codecov Report

Merging #826 (c4492fb) into master (931b95a) will decrease coverage by 0.19%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #826      +/-   ##
============================================
- Coverage     44.90%   44.70%   -0.20%     
+ Complexity      756      750       -6     
============================================
  Files           111      111              
  Lines          6527     6527              
  Branches        920      920              
============================================
- Hits           2931     2918      -13     
- Misses         3188     3195       +7     
- Partials        408      414       +6     

Impacted Files	Coverage Δ
...rc/main/kotlin/org/jitsi/jicofo/jibri/BaseJibri.kt	0.00% <0.00%> (ø)
...si/jicofo/conference/AbstractChannelAllocator.java	39.08% <0.00%> (-3.45%)	⬇️
...itsi/protocol/xmpp/AbstractOperationSetJingle.java	54.18% <0.00%> (-1.68%)	⬇️
...tsi/jicofo/conference/JitsiMeetConferenceImpl.java	43.70% <0.00%> (-0.91%)	⬇️

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