Remove locale dependent FileSystemException check (#21633)

In SourceFile, files that do not exist are expected and related
exceptions should be ignored. There are two relevant cases:

scala>
java.nio.file.Files.newInputStream(java.nio.file.FileSystems.getDefault().getPath("does-not-exist"))
    java.nio.file.NoSuchFileException: does-not-exist

scala>
java.nio.file.Files.newInputStream(java.nio.file.FileSystems.getDefault().getPath("regular-file-instead-of-directory/filename"))
java.nio.file.FileSystemException:
regular-file-instead-of-directory/filename: Not a directory

Ideally, other I/O errors would be propagated to the caller.

However, there is no reliable way to distinguish them based on the
exceptions alone. In particular, the message cannot be checked, because
it depends on the operating system and it is localized.

Revert the addition of the check and just accept this.

---

Resolves the following concrete bug:

0. Use an operating system that localizes strerror, like Debian.
1. Check out https://github.com/lichess-org/lila before
lichess-org/lila@8610e6f
2. `LANGUAGE=fr sbt compile`
3. ICE with `java.nio.file.FileSystemException:
lila/src/main/scala/future.scala: N'est pas un dossier`
([stacktrace](https://gist.github.com/ornicar/4a7b136d2220f4c1fd43e920956f67b4))

Not sure how to exercise locale dependent issues in a unit test.

---

cc @som-snytt in case I misinterpreted
159da44 and distinguishing other I/O
errors is not optional.

compiler/src/dotty/tools/dotc/util/SourceFile.scala

@@ -276,12 +276,11 @@ object SourceFile {
 
   def apply(file: AbstractFile | Null, codec: Codec): SourceFile =
     // Files.exists is slow on Java 8 (https://rules.sonarsource.com/java/tag/performance/RSPEC-3725),
-    // so cope with failure; also deal with path prefix "Not a directory".
+    // so cope with failure.
     val chars =
       try new String(file.toByteArray, codec.charSet).toCharArray
       catch
-        case _: NoSuchFileException => Array.empty[Char]
-        case fse: FileSystemException if fse.getMessage.endsWith("Not a directory") => Array.empty[Char]
+        case _: FileSystemException => Array.empty[Char]
 
     if isScript(file, chars) then
       ScriptSourceFile(file, chars)