dtl: handle errors in context api

Both `GET /eth/context/latest` and `GET /eth/context/blocknumber/:number`
rely on fetching a remote block to pull the timestamp, blocknumber and
blockhash from. If the fetched block is not found, then properties on
`null` will be attempted to be accessed which will result in a runtime
error. This commit adds handling for this case to prevent that kind of
bug.

.changeset/warm-news-punch.md

+---
+'@eth-optimism/data-transport-layer': patch
+---
+
+Handle case where the remote block isn't found for `GET /eth/context/latest` and `GET /eth/context/blocknumber/:number`

packages/data-transport-layer/src/services/server/service.ts

@@ -340,6 +340,9 @@ export class L1TransportServer extends BaseService<L1TransportServerOptions> {
         const blockNumber = Math.max(0, tip - this.options.confirmations)
 
         const block = await this.state.l1RpcProvider.getBlock(blockNumber)
+        if (block === null) {
+          throw new Error(`Cannot GET /eth/context/latest at ${blockNumber}`)
+        }
 
         return {
           blockNumber: block.number,
@@ -366,6 +369,10 @@ export class L1TransportServer extends BaseService<L1TransportServerOptions> {
         }
 
         const block = await this.state.l1RpcProvider.getBlock(number)
+        if (block === null) {
+          throw new Error(`Cannot GET /eth/context/blocknumber/${number}`)
+        }
+
         return {
           blockNumber: block.number,
           timestamp: block.timestamp,