Fix log messages when attempting to rebuild the plugin, as the previous messages were showing the incorrect output if `cmake` failed, or if `make` failed to execute.
Added output error messsages to the tmux plugin file so that if either
cmake or make fail, you can find out why.
For example, if cmake is missing, the output looks like this:
```
'cmake /usr/local/google/home/nhdaly/.tmux/plugins/tmux-mem-cpu-load' failed.
/usr/local/google/home/nhdaly/.tmux/plugins//tmux-mem-cpu-load/tmux-mem-cpu-load.plugin.tmux: line 23: cmake: command not found
```
If there is some error during make, the output looks like this:
```
tmux-mem-cpu-load failed to build.
make: <make output error message>
```
Now the plugin can be installed via tmux-plugins/tpm in addition to
being installed through zsh's antigen package manager.
It now only calls `make install` when installed through antigen; the
tmux plugin automatically compiles the binary, but leaves it in the
plugin directory.
This also allows skipping the "if exists" check, which means plugin
updates can also automatically rebuild any changes to the compiled part
of the plugin.
Updated the README to reflect this new install option.
This is part of Issue #43.
