Improve the handling of errors, in PartialEvaluator.loadFont, occuring in PartialEvaluator.preEvaluateFont (issue 12823)

Currently any errors thrown in `preEvaluateFont`, which is a *synchronous* method, will not be handled at all in the `loadFont` method and we were thus failing to return an `ErrorFont`-instance as intended here.

Also, add an *explicit* check in `PartialEvaluator.preEvaluateFont` to ensure that Type0-fonts always have a *valid* dictionary.

test/pdfs/.gitignore

@@ -70,6 +70,7 @@
 !issue8570.pdf
 !issue8697.pdf
 !issue8702.pdf
+!issue12823.pdf
 !issue8707.pdf
 !issue8798r.pdf
 !issue8823.pdf

test/test_manifest.json

@@ -1163,6 +1163,12 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue12823",
+       "file": "pdfs/issue12823.pdf",
+       "md5": "8391c63006ccc4b41c95de5e8c38deeb",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "bug1050040",
        "file": "pdfs/bug1050040.pdf",
        "md5": "9076b29bd157e2646b457f29a4472a07",