Internal error when crystal run fails to read source file

[HertzDevil]

Since #13853, crystal run fails with an internal error when the file name is invalid:

$ bin/crystal run /
Using compiled compiler at .build/crystal
Error reading file: Is a directory (IO::Error)
  from src/io/evented.cr:61:9 in 'unbuffered_read'
  from src/io/buffered.cr:82:16 in 'read'
  from src/io.cr:1179:12 in 'copy'
  from src/file.cr:740:11 in 'read'
  from src/file.cr:728:3 in 'read'
  from src/compiler/crystal/command.cr:618:38 in 'gather_sources'
  from src/compiler/crystal/command.cr:575:20 in 'create_compiler'
  from src/compiler/crystal/command.cr:363:11 in 'create_compiler:run:single_file'
  from src/compiler/crystal/command.cr:223:14 in 'run_command'
  from src/compiler/crystal/command.cr:111:7 in 'run'
  from src/compiler/crystal/command.cr:54:5 in 'run'
  from src/compiler/crystal/command.cr:53:3 in 'run'
  from src/compiler/crystal.cr:11:1 in '__crystal_main'
  from src/crystal/main.cr:129:5 in 'main_user_code'
  from src/crystal/main.cr:115:7 in 'main'
  from src/crystal/main.cr:141:3 in 'main'
  from /lib/x86_64-linux-gnu/libc.so.6 in '??'
  from /lib/x86_64-linux-gnu/libc.so.6 in '__libc_start_main'
  from /home/quinton/crystal/crystal/.build/crystal in '_start'
  from ???
Error: you've found a bug in the Crystal compiler. Please open an issue, including source code that will allow us to reproduce the bug: https://github.com/crystal-lang/crystal/issues

$ $ bin/crystal run /foo
Using compiled compiler at .build/crystal
Error opening file with mode 'r': '/foo': No such file or directory (File::NotFoundError)
  from src/crystal/system/unix/file.cr:12:7 in 'open'
  from src/file.cr:162:5 in 'new'
  from src/file.cr:714:5 in 'read'
  from src/file.cr:728:3 in 'read'
  from src/compiler/crystal/command.cr:618:38 in 'gather_sources'
  from src/compiler/crystal/command.cr:575:20 in 'create_compiler'
  from src/compiler/crystal/command.cr:363:11 in 'create_compiler:run:single_file'
  from src/compiler/crystal/command.cr:223:14 in 'run_command'
  from src/compiler/crystal/command.cr:111:7 in 'run'
  from src/compiler/crystal/command.cr:54:5 in 'run'
  from src/compiler/crystal/command.cr:53:3 in 'run'
  from src/compiler/crystal.cr:11:1 in '__crystal_main'
  from src/crystal/main.cr:129:5 in 'main_user_code'
  from src/crystal/main.cr:115:7 in 'main'
  from src/crystal/main.cr:141:3 in 'main'
  from /lib/x86_64-linux-gnu/libc.so.6 in '??'
  from /lib/x86_64-linux-gnu/libc.so.6 in '__libc_start_main'
  from /home/quinton/crystal/crystal/.build/crystal in '_start'
  from ???
Error: you've found a bug in the Crystal compiler. Please open an issue, including source code that will allow us to reproduce the bug: https://github.com/crystal-lang/crystal/issues

[straight-shoota]

I mentioned in #13853 that this is not finally polished. In particular, I'm blocked by #13852 or #13865 in order to proceed. Without either of those changes, the error message would be subpar or require extra effort on this particular call site (which I'd rather avoid).

[HertzDevil]

This should now be unblocked