print a stack if setupProxy.js doesn't parse

Created by: scottlamb

Before, npm start would say this:

Unexpected token '?'
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! ui@0.1.0 start: `react-scripts start`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the ui@0.1.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR!     /home/slamb/.npm/_logs/2021-03-04T20_50_38_776Z-debug.log

(The logfile isn't helpful; it doesn't even say the Unexpected token '?' part.)

After, the Unexpected token '?' line is replaced with the following:

/home/slamb/git/moonfire-nvr/ui/src/setupProxy.js:13
      target: process.env.PROXY_TARGET ?? "http://localhost:8080/",
                                        ^

SyntaxError: Unexpected token '?'
    at wrapSafe (internal/modules/cjs/loader.js:1054:16)
    at Module._compile (internal/modules/cjs/loader.js:1102:27)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1158:10)
    at Module.load (internal/modules/cjs/loader.js:986:32)
    at Function.Module._load (internal/modules/cjs/loader.js:879:14)
    at Module.require (internal/modules/cjs/loader.js:1026:19)
    at require (internal/modules/cjs/helpers.js:72:18)
    at Object.before (/home/slamb/git/moonfire-nvr/ui/node_modules/react-scripts/config/webpackDevServer.config.js:123:9)
    at Server.setupBeforeFeature (/home/slamb/git/moonfire-nvr/ui/node_modules/webpack-dev-server/lib/Server.js:455:18)
    at Object.before (/home/slamb/git/moonfire-nvr/ui/node_modules/webpack-dev-server/lib/Server.js:515:16)