micromamba run with missing env still attempts to execute the command

[jonashaag]

$ micromamba run -n xxx python
warning  libmamba 'root_prefix' set with default value: /Users/j/micromamba
critical libmamba Cannot activate, prefix does not exist at: /Users/j/micromamba/envs/xxx
/var/folders/8v/x6lkdp6110d5w7z73g_jfm_00000gn/T/mambafjhLFdy8hm4: line 5: exec: python: not found

Env is missing so cannot be activated. Still the command is attempted to be executed as can be seen from the exec error message. This can lead to confusing behaviour if the command to be executed exists outside the environment (eg. system Python in this case). We should just stop after the critical error.