Add a recoveryMode that suppresses errors from the Parser, and utilize it when searching for the main trailer in XRef_indexObjects (bug 1250079)

Instead of having `Parser_getObj` fail unconditionally for the referenced PDF file, this patch attempts to let searching for the main trailer continue even if there are errors.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1250079.

test/pdfs/.gitignore

@@ -99,6 +99,7 @@
 !bug911034.pdf
 !bug1108301.pdf
 !bug1157493.pdf
+!bug1250079.pdf
 !pdfjsbad1586.pdf
 !freeculture.pdf
 !issue6006.pdf

test/test_manifest.json

@@ -730,6 +730,13 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "bug1250079",
+       "file": "pdfs/bug1250079.pdf",
+       "md5": "a1dd21a70ae7097d96273e85a80b26ef",
+       "rounds": 1,
+       "link": false,
+       "type": "load"
+    },
     {  "id": "hudsonsurvey",
        "file": "pdfs/hudsonsurvey.pdf",
        "md5": "bf0e6576a7b6c2fe7485bce1b78e006f",