Watch API improvements

[smira]

Introduce new types of Watch events:

• Error to pass through errors, we need to use it in the gRPC wrapper, in the etcd state implementation; inmem implementation can use it to report buffer overrun
• Bootstrapped to report when initial contents are sent (only when BootstrapContents is requested)

Figure out how to introduce it without breaking clients... New Watch call? add options to the existing one?