bitcoin-tx: Reject + sign in nversion parsing

It would be confusing to specify the sign for an unsigned value here, so
reject it.

test/util/data/bitcoin-util-test.json

@@ -56,6 +56,12 @@
     "output_cmp": "blanktxv2.json",
     "description": "Creates a blank transaction when nothing is piped into bitcoin-tx (output in json)"
   },
+  { "exec": "./bitcoin-tx",
+    "args": ["-create", "nversion=+1"],
+    "return_code": 1,
+    "error_txt": "error: Invalid TX version requested",
+    "description": "Tests the check for invalid nversion value"
+  },
   { "exec": "./bitcoin-tx",
     "args": ["-create", "nversion=1foo"],
     "return_code": 1,