Return correct error codes in blockchain.cpp.

RPCs in blockchain.cpp were returning misleading or incorrect error
codes (for example getblock() returning RPC_INTERNAL_ERROR when the
block had been pruned). This commit fixes those error codes:

- RPC_INTERNAL_ERROR should not be returned for application-level
  errors, only for genuine internal errors such as corrupted data.
- RPC_METHOD_NOT_FOUND should not be returned in response to a
  JSON request for an existing method.

Those error codes have been replaced with RPC_MISC_ERROR or
RPC_INVALID_PARAMETER as appropriate.

qa/rpc-tests/p2p-acceptblock.py

@@ -197,11 +197,8 @@ class AcceptBlockTest(BitcoinTestFramework):
                 assert_equal(x['status'], "headers-only")
 
         # But this block should be accepted by node0 since it has more work.
-        try:
-            self.nodes[0].getblock(blocks_h3[0].hash)
-            print("Unrequested more-work block accepted from non-whitelisted peer")
-        except:
-            raise AssertionError("Unrequested more work block was not processed")
+        self.nodes[0].getblock(blocks_h3[0].hash)
+        print("Unrequested more-work block accepted from non-whitelisted peer")
 
         # Node1 should have accepted and reorged.
         assert_equal(self.nodes[1].getblockcount(), 3)
@@ -225,26 +222,17 @@ class AcceptBlockTest(BitcoinTestFramework):
                 tips[j] = next_block
 
         time.sleep(2)
-        for x in all_blocks:
-            try:
-                self.nodes[0].getblock(x.hash)
-                if x == all_blocks[287]:
-                    raise AssertionError("Unrequested block too far-ahead should have been ignored")
-            except:
-                if x == all_blocks[287]:
-                    print("Unrequested block too far-ahead not processed")
-                else:
-                    raise AssertionError("Unrequested block with more work should have been accepted")
+        # Blocks 1-287 should be accepted, block 288 should be ignored because it's too far ahead
+        for x in all_blocks[:-1]:
+            self.nodes[0].getblock(x.hash)
+        assert_raises_jsonrpc(-1, "Block not found on disk", self.nodes[0].getblock, all_blocks[-1].hash)
 
         headers_message.headers.pop() # Ensure the last block is unrequested
         white_node.send_message(headers_message) # Send headers leading to tip
         white_node.send_message(msg_block(tips[1]))  # Now deliver the tip
-        try:
-            white_node.sync_with_ping()
-            self.nodes[1].getblock(tips[1].hash)
-            print("Unrequested block far ahead of tip accepted from whitelisted peer")
-        except:
-            raise AssertionError("Unrequested block from whitelisted peer not accepted")
+        white_node.sync_with_ping()
+        self.nodes[1].getblock(tips[1].hash)
+        print("Unrequested block far ahead of tip accepted from whitelisted peer")
 
         # 5. Test handling of unrequested block on the node that didn't process
         # Should still not be processed (even though it has a child that has more