Issue #135 - Handle better the diff viewer not having coverage data

Sometimes, we would need to visit the diff viewer for old changesets.
Unfortunately, the backend would not have data available and the viewer
would fail ungracefully.

Before this, the console would be full of errors even if the UI
would not fail dramatically.

src/components/diffViewer.jsx

@@ -2,7 +2,6 @@ import { Link } from 'react-router-dom';
 import DiffFile from './diffFile';
 import CoverageMeta from './coverageMeta';
 import CoverageFooter from './coverageFooter';
-import settings from '../settings';
 
 const DiffViewer = ({
   appError, changeset, coverage, parsedDiff,
@@ -12,7 +11,7 @@ const DiffViewer = ({
       <Link to="/" href="/">Return to main page</Link>
     </div>
     <span className="error_message">{appError}</span>
-    {(coverage && coverage.summary !== settings.STRINGS.PENDING) && (
+    {(coverage && coverage.show) && (
       <div>
         <CoverageMeta
           changeset={changeset}

src/containers/diffViewer.jsx

@@ -29,9 +29,11 @@ export default class DiffViewerContainer extends Component {
   constructor(props) {
     super(props);
     this.state = {
-      appError: undefined,
-      coverage: undefined,
+      appError: '',
+      changeset: {},
+      coverage: {},
       parsedDiff: [],
+      supportedExtensions: [],
     };
   }
 
@@ -53,12 +55,14 @@ export default class DiffViewerContainer extends Component {
   async fetchSetCoverageData(node) {
     try {
       const coverage = await getChangesetCoverage(node);
+      let appError = '';
       if (coverage.summary === settings.STRINGS.PENDING) {
-        this.setState({
-          appError: 'The coverage data is still pending. Try again later.',
-        });
+        appError = 'The coverage data is still pending. Please try again later.';
       }
-      this.setState({ coverage });
+      if (!coverage.show) {
+        appError = 'The coverage backend has had an internal error.';
+      }
+      this.setState({ appError, coverage });
     } catch (error) {
       console.error(error);
       this.setState({
@@ -94,10 +98,17 @@ export default class DiffViewerContainer extends Component {
 
   render() {
     const {
-      appError, changeset, coverage, parsedDiff, supportedExtensions,
+      appError,
+      changeset,
+      coverage,
+      parsedDiff,
+      supportedExtensions,
     } = this.state;
-    const onlySupportedExtensions = filterUnsupportedExtensions(parsedDiff, supportedExtensions);
-    const sortedDiff = sortByPercent(onlySupportedExtensions, coverage);
+    let sortedDiff = [];
+    if (coverage && coverage.show) {
+      const onlySupportedExtensions = filterUnsupportedExtensions(parsedDiff, supportedExtensions);
+      sortedDiff = sortByPercent(onlySupportedExtensions, coverage);
+    }
 
     return (
       <DiffViewer