fix: handle unexpected error when generating code coverage

The issue surfaced itself after d970028. The EEXIST error from concurrent attempts to create the same directory were previously silently swallowed, but started to show up once the call was properly synchronized.

The EEXIST will now result in `promiseComplete` being rejected and reported as `unhandledRejection` because it is awaited in the `onExit` callback. The unhandled rejection is then picked up by `karma` [here](https://github.com/karma-runner/karma/blob/c985155a4eac95c525e1217e98d4013ac5f53305/lib/server.js#L395) triggering [the close logic](https://github.com/karma-runner/karma/blob/c985155a4eac95c525e1217e98d4013ac5f53305/lib/server.js#L392) which (among other things) will trigger `onExit` callback causing an infinite loop.

The local fix is to handle the rejected promise directly and report failure to karma by passing a non-zero exit code.

lib/reporter.js

@@ -290,15 +290,20 @@ var CoverageReporter = function (rootConfig, helper, logger, emitter) {
   }
 
   this.onExit = async function (done) {
-    const results = await promiseComplete
-    if (results && results.exitCode === 1) {
-      done(results.exitCode)
-      return
-    }
-    if (typeof config._onExit === 'function') {
-      config._onExit(done)
-    } else {
-      done()
+    try {
+      const results = await promiseComplete
+      if (results && results.exitCode === 1) {
+        done(results.exitCode)
+        return
+      }
+      if (typeof config._onExit === 'function') {
+        config._onExit(done)
+      } else {
+        done()
+      }
+    } catch (e) {
+      log.error(`Unexpected error while generating coverage report: ${e}`)
+      done(1)
     }
   }
 }