Fall back to credentials when ssh keys fail on sftp upload

When trying to upload to our private ftp update site from an up-to-date Fiji installation (on Windows), I now get the following exception:

```
[ERROR] com.jcraft.jsch.JSchException: invalid privatekey: [B@5584d17
at com.jcraft.jsch.KeyPair.load(KeyPair.java:664)
at com.jcraft.jsch.KeyPair.load(KeyPair.java:561)
at com.jcraft.jsch.IdentityFile.newInstance(IdentityFile.java:40)
at com.jcraft.jsch.JSch.addIdentity(JSch.java:406)
at com.jcraft.jsch.JSch.addIdentity(JSch.java:366)
at net.imagej.plugins.uploaders.ssh.SSHSessionCreator.connect(SSHSessionCreator.java:80)
at net.imagej.plugins.uploaders.ssh.SSHSessionCreator.getSession(SSHSessionCreator.java:251)
at net.imagej.plugins.uploaders.ssh.SSHFileUploader.login(SSHFileUploader.java:81)
at net.imagej.updater.FilesUploader.login(FilesUploader.java:410)
at net.imagej.ui.swing.updater.UpdaterFrame.upload(UpdaterFrame.java:801)
at net.imagej.ui.swing.updater.UpdaterFrame$5$1.run(UpdaterFrame.java:309)
```

Start in SSHSessionCreator.connect at line 80, then follow the login flow through SSHSessionCreator.getSession and SSHFileUploader.login. Reproduce the Windows SFTP upload with the failing private key and verify that the uploader falls back to credentials and completes the upload when key loading fails.

Written by the indexing model from the issue text.