Error out when the shell script to be executed cannot be written to disk

[josegonzalez]

If you run out of disk space and fail to write the script, we'll get an error like:

/home/dokku/.basher/bash: line 1: main: command not found

This is because of us not checking the error on the write: https://github.com/progrium/go-basher/blob/master/basher.go#L225

What we should do is attempt to sync the file to disk, and fail if it can't. I believe the write failure should show more context than the current error that appears later in the process (when calling the script).

Refs dokku/dokku#5764