deal with non-existing packages in destination channel

[k-dominik]

currently publish-conda-stack will fail for packages that do not exist in the destination channel when doing conda/mamba search

INFO: mamba search --json --full-name --override-channels --channel ilastik-forge some-package                        
Traceback (most recent call last):                                    
  File "/.../miniconda3/bin/publish-conda-stack", line 10, in <module>                                                       
    sys.exit(main())                                                  
  File "/.../miniconda3/lib/python3.9/site-packages/publish_conda_stack/core.py", line 227, in main                          
    raise e                                                                                                                                 
  File "/.../miniconda3/lib/python3.9/site-packages/publish_conda_stack/core.py", line 223, in main                          
    status = build_and_upload_recipe(spec, shared_config, conda_bld_config)                                                                 
  File "/.../miniconda3/lib/python3.9/site-packages/publish_conda_stack/core.py", line 390, in build_and_upload_recipe       
    packages_found = check_already_exists(c_pkg_names, shared_config) 
  File "/.../miniconda3/lib/python3.9/site-packages/publish_conda_stack/core.py", line 527, in check_already_exists          
    search_results_text = subprocess.check_output(search_cmd).decode()                                                                      
  File "/.../miniconda3/lib/python3.9/subprocess.py", line 424, in check_output                                              
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,                                                                        
  File "/.../miniconda3/lib/python3.9/subprocess.py", line 528, in run                                                       
    raise CalledProcessError(retcode, process.args,                                                                                         
subprocess.CalledProcessError: Command '['mamba', 'search', '--json', '--full-name', '--override-channels', '--channel', 'ilastik-forge', 'i
some-package']' returned non-zero exit status 1.