Pass the ignoreErrors API option to the FontFaceObject constructor, and utilize it in getPathGenerator to ignore missing glyphs

Obviously it's still not possible to render non-embedded fonts as paths, but in this way the rest of the page will at least be allowed to continue rendering.

*Please note:* Including the 14 standard fonts in PDF.js probably wouldn't be *that* difficult to implement. (I'm not a lawyer, but the fonts from PDFium could probably be used given their BSD license.)
However, the main blocker ought to be the total size of the necessary font data, since I cannot imagine people being OK with shipping ~5 MB of (additional) font data with Firefox. (Based on the reactions when the CMap files were added, and those are only ~1 MB in size.)

src/display/api.js

@@ -1884,6 +1884,7 @@ var WorkerTransport = (function WorkerTransportClosure() {
             var font = new FontFaceObject(exportedData, {
               isEvalSupported: params.isEvalSupported,
               disableFontFace: params.disableFontFace,
+              ignoreErrors: params.ignoreErrors,
               fontRegistry,
             });
             var fontReady = (fontObjs) => {

src/display/font_loader.js

@@ -338,6 +338,7 @@ var IsEvalSupportedCached = {
 var FontFaceObject = (function FontFaceObjectClosure() {
   function FontFaceObject(translatedData, { isEvalSupported = true,
                                             disableFontFace = false,
+                                            ignoreErrors = false,
                                             fontRegistry = null, }) {
     this.compiledGlyphs = Object.create(null);
     // importing translated data
@@ -346,6 +347,7 @@ var FontFaceObject = (function FontFaceObjectClosure() {
     }
     this.isEvalSupported = isEvalSupported !== false;
     this.disableFontFace = disableFontFace === true;
+    this.ignoreErrors = ignoreErrors === true;
     this.fontRegistry = fontRegistry;
   }
   FontFaceObject.prototype = {
@@ -390,7 +392,19 @@ var FontFaceObject = (function FontFaceObjectClosure() {
         return this.compiledGlyphs[character];
       }
 
-      let cmds = objs.get(this.loadedName + '_path_' + character), current;
+      let cmds, current;
+      try {
+        cmds = objs.get(this.loadedName + '_path_' + character);
+      } catch (ex) {
+        if (!this.ignoreErrors) {
+          throw ex;
+        }
+        warn(`getPathGenerator - ignoring character: "${ex}".`);
+
+        return this.compiledGlyphs[character] = function(c, size) {
+          // No-op function, to allow rendering to continue.
+        };
+      }
 
       // If we can, compile cmds into JS for MAXIMUM SPEED...
       if (this.isEvalSupported && IsEvalSupportedCached.value) {

test/pdfs/issue4244.pdf.link

@@ -0,0 +1 @@
+https://web.archive.org/web/20180613082417/https://tcpdf.org/files/examples/example_026.pdf

test/test_manifest.json

@@ -2267,6 +2267,13 @@
       "rounds": 1,
       "type": "eq"
     },
+    {  "id": "issue4244",
+       "file": "pdfs/issue4244.pdf",
+       "md5": "26845274a32a537182ced1fd693a38b2",
+       "rounds": 1,
+       "link": true,
+       "type": "eq"
+    },
     {  "id": "preistabelle",
       "file": "pdfs/preistabelle.pdf",
       "md5": "d2f0b2086160d4f3d325c79a5dc1fb4d",