airline#system#util for nvim used to fall back to 'system' implementation on
command error. This behavior caused conflict with other plugins if 'util' was
executed with a failing command as part of a ShellCmdPost event.
This commit makes 'util' interpret command error as persistent and not call
'system' in such a case.
This commit fixes#1317.
This commit makes branch.vim use neovim's async jobs instead of a system()
function. This way we avoid the v:shell_error overwrite bug and allow live
updates of the untracked status.
airline#extensions#load() tries to load all extensions,
the user has specified in g:airline_extensions.
However, if e.g. bufferline is requested to be loaded, but
is not installed, it will throw an error
E117: Unknown function: airline#extensions#bufferline#init
Therefore, catch E117 and show a warning.
closes#1127
