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/p2p_node_network_limited.py

@@ -102,10 +102,10 @@ class NodeNetworkLimitedTest(BitcoinTestFramework):
         tip_height = pruned_node.getblockcount()
         limit_buffer = 2
         # Prevent races by waiting for the tip to arrive first
-        self.wait_until(lambda: not try_rpc(-1, "Block not found", full_node.getblock, pruned_node.getbestblockhash()))
+        self.wait_until(lambda: not try_rpc(-1, "Block not available (not fully downloaded)", full_node.getblock, pruned_node.getbestblockhash()))
         for height in range(start_height_full_node + 1, tip_height + 1):
             if height <= tip_height - (NODE_NETWORK_LIMITED_MIN_BLOCKS - limit_buffer):
-                assert_raises_rpc_error(-1, "Block not found on disk", full_node.getblock, pruned_node.getblockhash(height))
+                assert_raises_rpc_error(-1, "Block not available (not fully downloaded)", full_node.getblock, pruned_node.getblockhash(height))
             else:
                 full_node.getblock(pruned_node.getblockhash(height))  # just assert it does not throw an exception