Add test for empty suites exit code
Document exit code for improperly formatted suites
Use a promised version of fs.readdir when loading suites

Fixes #112
Improve error message for empty suites
Test error message on empty suites
Uniform exit codes for malformed suites
Ensure browser is quit if suite is malformed
Simplify error handling of unreadable suites
Allow time for browser startup in empty suite integration test
Handle loadAllFiles exceptions (fixes #112)

Ensure fs.readdir is called in the promise chain
Cover with integration test due to lack of SuiteLoader test suite

* handle-loadallfiles-exceptions:
  Allow time for browser startup in empty suite integration test
  Simplify error handling of unreadable suites
  Ensure browser is quit if suite is malformed
  Uniform exit codes for malformed suites
  Test error message on empty suites
  Improve error message for empty suites
  Use a promised version of fs.readdir when loading suites
  Document exit code for improperly formatted suites
  Add test for empty suites exit code