rpc: Improve getblock / getblockstats error when only header is available.

This improves the error message of the getblock and getblockstats rpc and prevents calls to
ReadRawBlockFromDisk(), which are unnecessary if we know
from the header nStatus field that the block is not available.

test/functional/rpc_getblockfrompeer.py

@@ -58,7 +58,7 @@ class GetBlockFromPeerTest(BitcoinTestFramework):
         self.log.info("Node 0 should only have the header for node 1's block 3")
         x = next(filter(lambda x: x['hash'] == short_tip, self.nodes[0].getchaintips()))
         assert_equal(x['status'], "headers-only")
-        assert_raises_rpc_error(-1, "Block not found on disk", self.nodes[0].getblock, short_tip)
+        assert_raises_rpc_error(-1, "Block not available (not fully downloaded)", self.nodes[0].getblock, short_tip)
 
         self.log.info("Fetch block from node 1")
         peers = self.nodes[0].getpeerinfo()