invalid syntax when a TMPL variable is undefined

[joe-p]

Currently when a TMPL variable is undefined the client will throw this error from algod when trying to compile:

URLTokenBaseHTTPError: Network request error. Received status 400 (Bad Request): 1 error: 6:58: strconv.ParseUint: parsing "TMPL_NFD_REGISTRY_APP_ID": invalid syntax

Instead, we should check the constant blocks for any TMPL_ and generate a more human-friendly error about the undefined TMPL variable.