[DLST-463] fix dry-run and .temp directory bugs

[goldsmithb]

Jira Ticket Link

1. Fixes a bug where .temp directories were not being deleted in dry-run mode (the call to the rm_temp_dirs method wasn't early enough in #email_and_exit)
2. Fixes a bug where a .temp directory version of the uploaded dissertation directory already exists in the preservation location (this shouldn't happen due to the bugfix for point 1, but now we check for and handle this possibility)

Other changes:

• (In SftpClient class) the method dissertations_dir_already_exists? has been replaced with the method check_dissertations_dir_already_exists - instead of returning a boolean value and having the caller raise an error if the result is true, now the function raises an error if the directory has already been downloaded, and returns an array with the string names of each directory to be downloaded (this value is immediately discarded for now, but could be used for logging/debugging purposes)

[elohanlon]

Looks good! Just left one minor suggestion about a method name.