Catch RuntimeException: mb_convert_encoding(): Illegal character encoding specified at PhabricatorTextDocumentEngine.php:73

Summary:
When given `$encoding` is invalid, catch the exception to show a proper error message and make the server logs provide more hints.

```
EXCEPTION: (RuntimeException) mb_convert_encoding(): Illegal character encoding specified at [<arcanist>/src/error/PhutilErrorHandler.php:261]
#0 <freebsd#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
freebsd#1 <freebsd#2> mb_convert_encoding(string, string, string) called at [<phabricator>/src/applications/files/document/PhabricatorTextDocumentEngine.php:73]
```

Closes T15624

Test Plan: Open a URL which passes a bogus encoding value as parameter, like `/source/somerepository/browse/master/README.md?as=source&encode=TROLOLOL`

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: Sten, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15624

Differential Revision: https://we.phorge.it/D25418

Author: aklapper

src/applications/files/document/PhabricatorTextDocumentEngine.php

@@ -70,10 +70,16 @@ protected function loadTextData(PhabricatorDocumentRef $ref) {
     $encoding = $this->getEncodingConfiguration();
     if ($encoding !== null) {
       if (function_exists('mb_convert_encoding')) {
-        $content = mb_convert_encoding($content, 'UTF-8', $encoding);
-        $this->encodingMessage = pht(
-          'This document was converted from %s to UTF8 for display.',
-          $encoding);
+        try {
+          $content = mb_convert_encoding($content, 'UTF-8', $encoding);
+          $this->encodingMessage = pht(
+            'This document was converted from %s to UTF8 for display.',
+            $encoding);
+        } catch (Throwable $ex) {
+          $this->encodingMessage = pht(
+            'Unable to convert from requested encoding %s to UTF8.',
+            $encoding);
+        }
       } else {
         $this->encodingMessage = pht(
           'Unable to perform text encoding conversion: mbstring extension '.