Remove locale dependent FileSystemException check

[niklasf]

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.

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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)

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

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