Add beacon.watch (#3362)

> This is currently a WIP and all features are subject to alteration or removal at any time.

## Overview

The successor to #2873.

Contains the backbone of `beacon.watch` including syncing code, the initial API, and several core database tables.

See `watch/README.md` for more information, requirements and usage.

watch/src/database/error.rs

@@ -0,0 +1,55 @@
+use bls::Error as BlsError;
+use diesel::result::{ConnectionError, Error as PgError};
+use eth2::SensitiveError;
+use r2d2::Error as PoolError;
+use std::fmt;
+use types::BeaconStateError;
+
+#[derive(Debug)]
+pub enum Error {
+    BeaconState(BeaconStateError),
+    Database(PgError),
+    DatabaseCorrupted,
+    InvalidSig(BlsError),
+    PostgresConnection(ConnectionError),
+    Pool(PoolError),
+    SensitiveUrl(SensitiveError),
+    InvalidRoot,
+    Other(String),
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{:?}", self)
+    }
+}
+
+impl From<BeaconStateError> for Error {
+    fn from(e: BeaconStateError) -> Self {
+        Error::BeaconState(e)
+    }
+}
+
+impl From<ConnectionError> for Error {
+    fn from(e: ConnectionError) -> Self {
+        Error::PostgresConnection(e)
+    }
+}
+
+impl From<PgError> for Error {
+    fn from(e: PgError) -> Self {
+        Error::Database(e)
+    }
+}
+
+impl From<PoolError> for Error {
+    fn from(e: PoolError) -> Self {
+        Error::Pool(e)
+    }
+}
+
+impl From<BlsError> for Error {
+    fn from(e: BlsError) -> Self {
+        Error::InvalidSig(e)
+    }
+}