R1:2adc36ba0b8d

Correctly identify more SSH private key problems as "formatting" or "passphrase" related

Summary:
Ref T13454. Fixes T13006. When a user provide us with an SSH private key and (possibly) a passphrase:

# Try to verify that they're correct by extracting the public key.
# If that fails, try to figure out why it didn't work.

Our success in step (2) will vary depending on what the problem is, and we may end up falling through to a very generic error, but the outcome should generally be better than the old…