Log Errors from SlaveComputer._connect

[jglick]

When an (outbound) agent fails to launch, we expect any errors to be printed to the agent log. This is done by _connect, since (unlike the case in #7284) this returns a Future and a caller of Computer.connect may or may wait for it (get; most do not). While this checked for a variety of checked and unchecked exceptions, it failed to account for Errors.

Testing done

Discovered while investigating a (proprietary) test flake. Without the patch, when launching fails (randomly), nothing appears in the log; in fact slave.log is not created at all. With the patch, the agent log shows the error, in this case

ERROR: Unexpected error in launching an agent. This is probably a bug in Jenkins
java.lang.ClassNotFoundException: org.bouncycastle.crypto.prng.RandomGenerator
	at java.base/java.net.URLClassLoader.findClass(URLClassLoader.java:476)
	at jenkins.util.URLClassLoader2.findClass(URLClassLoader2.java:35)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:594)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:527)
Also:   java.lang.Throwable: launched here
	at hudson.slaves.SlaveComputer._connect(SlaveComputer.java:286)
	at hudson.model.Computer.connect(Computer.java:448)
	at …
Caused: java.lang.NoClassDefFoundError: org/bouncycastle/crypto/prng/RandomGenerator
	at org.apache.sshd.common.util.security.bouncycastle.BouncyCastleRandomFactory.create(BouncyCastleRandomFactory.java:43)
	at org.apache.sshd.common.util.security.bouncycastle.BouncyCastleRandomFactory.create(BouncyCastleRandomFactory.java:28)
	at org.apache.sshd.common.random.SingletonRandomFactory.<init>(SingletonRandomFactory.java:38)
	at org.apache.sshd.common.BaseBuilder.fillWithDefaultValues(BaseBuilder.java:166)
	at org.apache.sshd.client.ClientBuilder.fillWithDefaultValues(ClientBuilder.java:110)
	at org.apache.sshd.client.ClientBuilder.fillWithDefaultValues(ClientBuilder.java:54)
	at org.apache.sshd.common.BaseBuilder.build(BaseBuilder.java:273)
	at org.apache.sshd.client.ClientBuilder.build(ClientBuilder.java:161)
	at com.cloudbees.jenkins.plugins.sshslaves.PluginImpl.sshClient(PluginImpl.java:23)
	at com.cloudbees.jenkins.plugins.sshslaves.SSHLauncher.openConnection(SSHLauncher.java:326)
	at com.cloudbees.jenkins.plugins.sshslaves.SSHLauncher.launch(SSHLauncher.java:153)
	at hudson.slaves.SlaveComputer.lambda$_connect$0(SlaveComputer.java:297)
	at …

(Never mind the root cause for purposes of this PR.)

Proposed changelog entries

• More consistently report errors launching outbound agents.

Proposed upgrade guidelines

N/A

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[NotMyFault]

/label ready-for-merge

---

This PR is now ready for merge. We will merge it after ~24 hours if there is no negative feedback.
Please see the merge process documentation for more information about the merge process.
Thanks!