Better error reporting. (Prompted by `mean register` and `mean init`)

[BrockMcKean]

Several different issues that have surfaced recently have had little to no evidence initially of their root cause including mean register username length and mean init package installation (differences between bower dependencies on install and postinstall). Unfortunately the actual cause for mean init errors for installing packages with postinstall is still unknown, and if a user were to try to create a username over 15 characters, it wouldn't give them a very informative message.