Update the chainparams update scripts to NOT touch scalenet chainwork

Summary
---

In recent work in MR !1173, we accidentally almost updated the scalenet
chain params.  Since scalenet is intended to reorg back to block 10,000
in the future, it is not a good idea to update the chain params for this
network since it would make reorging problematic.

This MR adds a sys.exit() call to the `make_chainparams.py` script if it
encounters scalenet.  A dev will need to manually edit that script if
they wish to actually update scalenet.

This extra safety feature avoids the situation where someone may
inadvertently update scalenet's chain params, commit the change to
master, then publish a release with the updated chain params, making
reorgs of scalenet troublesome.

This MR closes issue Bitcoin-ABC#293

Test Plan
---

1. Review
2. Try it out:

- `cd devtools/chainparams`

- Run `bitcoind` on any network **OTHER** than scalenet.
- `./make_chainparams.py -a host:port -c /path/to/notscalenet.conf` <--
  this should output some chain params (2 lines of hex)

- Run `bitcoind` **OON** scalenet.
- `./make_chainparams.py -a host:port -c /path/to/scalenet.conf` <--
  this should output an error message

contrib/devtools/chainparams/README.md

@@ -32,6 +32,12 @@ bitcoind --scalenet
 python3 make_chainparams.py -a 127.0.0.1:38332 > chainparams_scalenet.txt
 ```
 
+**Note**: Scalenet should not be updated since it already has the chainparams it
+needs to be reorged back to height 10,000.  Without manually editing to comment-out
+the sys.exit call, the above script will exit with an error message if executed
+against a `bitcoind` that is on scalenet.
+
+
 ## Build C++ Header File
 ```
 python3 generate_chainparams_constants.py . > ../../../src/chainparamsconstants.h

contrib/devtools/chainparams/make_chainparams.py

@@ -31,6 +31,10 @@ def get_chainparams(rpc_caller, block):
         chain = Chain.TestNet4
     elif chaininfo['chain'] == 'scale':
         chain = Chain.ScaleNet
+        # Comment-out the below to actually update chain params for scalenet
+        sys.exit("ScaleNet chainparams should not be updated. See BCHN issue "
+                 "#293. If you really wish to proceed anyway, then please "
+                 "edit this script to comment-out this line of code.")
     else:
         raise NotImplementedError