Better error handling for compile errors

[aileen]

When running into an Handlebars error, GScan doesn't really tell you what went wrong.

GScan renders an error like that:

Uh-Oh, There was an error.
Cannot read property 'push' of undefined

TypeError: Cannot read property 'push' of undefined at /opt/gscan/releases/2018-09-06T12:20:32.572Z/lib/format.js:60:38 at /opt/gscan/shared/node_modules/lodash/lodash.js:4925:15 at baseForOwn (/opt/gscan/shared/node_modules/lodash/lodash.js:3010:24) at /opt/gscan/shared/node_modules/lodash/lodash.js:4894:18 at Function.forEach (/opt/gscan/shared/node_modules/lodash/lodash.js:9342:14) at Object.format (/opt/gscan/releases/2018-09-06T12:20:32.572Z/lib/format.js:52:7) at doRender (/opt/gscan/releases/2018-09-06T12:20:32.572Z/app/index.js:80:30) at Layer.handle [as handle_request] (/opt/gscan/shared/node_modules/express/lib/router/layer.js:95:5) at next (/opt/gscan/shared/node_modules/express/lib/router/route.js:137:13) at /opt/gscan/releases/2018-09-06T12:20:32.572Z/app/index.js:68:32 at PassThroughHandlerContext.finallyHandler (/opt/gscan/shared/node_modules/bluebird/js/release/finally.js:56:23) at PassThroughHandlerContext.tryCatcher (/opt/gscan/shared/node_modules/bluebird/js/release/util.js:16:23) at Promise._settlePromiseFromHandler (/opt/gscan/shared/node_modules/bluebird/js/release/promise.js:512:31) at Promise._settlePromise (/opt/gscan/shared/node_modules/bluebird/js/release/promise.js:569:18) at Promise._settlePromise0 (/opt/gscan/shared/node_modules/bluebird/js/release/promise.js:614:10) at Promise._settlePromises (/opt/gscan/shared/node_modules/bluebird/js/release/promise.js:693:18)

Which was - in this case - caused by opening with {{primary_author}}, but closing with {{/author}}. This should be detected and better communicated.