Need a SerializableThrowable for PAssert to capture the point of an assertion

For the regular Error class in Java, its stack trace is a transient attribute, so it's not serialized by any framework. We need a class that allows us to serialize these AssertionErrors to have them in PCollections.

Imported from Jira [BEAM-1898](https://issues.apache.org/jira/browse/BEAM-1898). Original Jira may contain additional context.
Reported by: pabloem.

Start by locating PAssert and the existing throwable or assertion handling, then inspect how failures are placed in PCollections. Confirm how Java Error stack traces behave during serialization; done means AssertionErrors can be serialized while retaining the assertion point in the resulting PCollections.

Written by the indexing model from the issue text.