change error message when stream is not found

[aler9]

Switch from 'no one is publishing to path' to 'no stream is available on path' since the stream might be provided from a static source too.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 57.27%. Comparing base (c692f3b) to head (912bec9).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/servers/rtsp/conn.go	0.00%	1 Missing ⚠️
internal/servers/rtsp/session.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4301      +/-   ##
==========================================
- Coverage   57.29%   57.27%   -0.02%     
==========================================
  Files         203      203              
  Lines       20591    20591              
==========================================
- Hits        11797    11794       -3     
- Misses       7904     7907       +3     
  Partials      890      890              

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