Catch error results from execute() and don't implode on non-arrays.

Needs proper error handling
proposed fix for https://github.com/bolt/bolt/issues/1512
add lowlevel check for existence of writable extensions directory
Merge pull request #1564 from GawainLynch/hotfix/CommandRunner-implodes

Catch error results from execute() and don't implode on non-arrays.
Merge pull request #1567 from jadwigo/master

proposed fix for https://github.com/bolt/bolt/issues/1512
Merge pull request #1568 from rossriley/extensions-lowlevel-check

Add lowlevel check for existence of writable extensions directory