Show errors of install_plugins

This PR makes plugin installation errors explicit.

Why?

I had a typo in my TMUX_PLUGIN_MANAGER_PATH which made it empty. With this TPM tries to download the plugins to / which it won't have permission to.

However before this PR, running ctrl+X and I won't show any error and I'm left wondering where the problem might be, until I finally looked at TPM code, removed the discard of the stderr and got the following:

/ is not writable!                                                                                                                                                                       [0/0]
Installing "tpm"
  "tpm" download fail
Installing "vim-tmux-navigator"
  "vim-tmux-navigator" download fail
Installing "tmux-themepack"
  "tmux-themepack" download fail
Installing "tmux-resurrect"
  "tmux-resurrect" download fail
Installing "tmux-fingers"
  "tmux-fingers" download fail
TMUX environment reloaded.
Done, press ENTER to continue.

Which then immediately pointed me to the variable. Imo making errors explicit, at least these errors where user action is required makes for a better user experience.

Fixing the TMUX_PLUGIN_MANAGER_PATH solved the issue, but it would have been much faster if I had the error message to start with

tmux-plugins / tpm

Show errors of install_plugins #178

Why?