Slow responses or failures: "UnhandledPromiseRejectionWarning" in logs

[allthesignals]

UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2190)
2019-07-04T07:14:30.762640861Z app[web.1]: (node:12) UnhandledPromiseRejectionWarning: FetchError: invalid json response body at https://planninglabs.carto.com/api/v1/map reason: Unexpected token < in JSON at position 0
2019-07-04T07:14:30.762671860Z app[web.1]: at /app/node_modules/node-fetch/lib/index.js:239:32
2019-07-04T07:14:30.762679740Z app[web.1]: at
2019-07-04T07:14:30.762685290Z app[web.1]: at process._tickCallback (internal/process/next_tick.js:189:7)
2019-07-04T07:14:30.762692350Z app[web.1]: (node:12) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2192)
2019-07-04T07:16:53.746479428Z app[web.1]: (node:12) UnhandledPromiseRejectionWarning: FetchError: invalid json response body at https://planninglabs.carto.com/api/v1/map reason: Unexpected token < in JSON at position 0
2019-07-04T07:16:53.746585972Z app[web.1]: at /app/node_modules/node-fetch/lib/index.js:239:32
2019-07-04T07:16:53.746601401Z app[web.1]: at
2019-07-04T07:16:53.746610628Z app[web.1]: at process._tickCallback (internal/process/next_tick.js:189:7)
2019-07-04T07:16:53.746671457Z app[web.1]: (node:12) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2194)
2019-07-04T07:17:38.989221770Z app[web.1]: (node:12) UnhandledPromiseRejectionWarning: FetchError: invalid json response body at https://planninglabs.carto.com/api/v1/map reason: Unexpected token < in JSON at position 0
2019-07-04T07:17:38.989294113Z app[web.1]: at /app/node_modules/node-fetch/lib/index.js:239:32
2019-07-04T07:17:38.989311456Z app[web.1]: at
2019-07-04T07:17:38.989321816Z app[web.1]: at process._tickCallback (internal/process/next_tick.js:189:7)

[allthesignals]

We should confirm that the delay comes from some issue with Carto. Assuming this is true, it probably has to do with there being too many requests for tile templates:

ZoLa, for example, requests about 17 tile templates from Carto per request, and the API makes no effort to roll up these into a single tile template.

One possible way to address this is to have the app round up all requested sources into a single request by collating all the relevant 'source-layers' properties. This easy fix is worth investigating.