Create pluggble server-side UncaughtExceptionHandler

**Is your feature request related to a problem? Please describe.**

Accumulo 2.1.x included a default non-configurable UncaughtExceptionHandler that terminates the VM on Error. There are some Error's that are not fatal, like a NoClassNotFoundError. With the use of ContextClassLoader's it's possible for an administrator to incorrectly configure the context, leading to a NoClassNotFoundError at scan time, which would cause the termination of one or more TabletServers. While this could be rare, the impact could be huge.

**Describe the solution you'd like**
A pluggable server-side UncaughtExceptionHandler

Start by locating the existing server-side UncaughtExceptionHandler and how TabletServers handle scan-time failures. Review the ContextClassLoader configuration path and determine the intended pluggable behavior before implementation. Done means the server-side handler can be configured and the relevant exception behavior is covered by tests.

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