core, eth, internal: Added `debug_getBadBlocks()` method (#3654)

* core,eth,internal: Added `debug_getBadBlocks()` method

When bad blocks are discovered, these are stored within geth.
An RPC-endpoint makes them availablewithin the `debug`
namespace. This feature makes it easier to discover network forks.

```

* core, api: go format + docs

* core/blockchain: Documentation, fix minor nitpick

* core: fix failing blockchain test

1 files changed, 6 insertions, 0 deletions

diff --git a/eth/api.go b/eth/api.go
index 07df0b79e..f38c0a6b6 100644
--- a/eth/api.go
+++ b/eth/api.go
@@ -566,3 +566,9 @@ func (api *PrivateDebugAPI) Preimage(ctx context.Context, hash common.Hash) (hex
 	db := core.PreimageTable(api.eth.ChainDb())
 	return db.Get(hash.Bytes())
 }
+
+// GetBadBLocks returns a list of the last 'bad blocks' that the client has seen on the network
+// and returns them as a JSON list of block-hashes
+func (api *PrivateDebugAPI) GetBadBlocks(ctx context.Context) ([]core.BadBlockArgs, error) {
+	return api.eth.BlockChain().BadBlocks()
+}