main: Flush file in place of closing it, also do error reporting

Apparently on travis OS X systems it crashes when cleaning up streams with
stdout closed:

    (lldb) bt all
    * thread #1: tid = 0x0000, 0x00007fff8703df06 libsystem_kernel.dylib`__pthread_kill + 10, stop reason = signal SIGSTOP
      * frame #0: 0x00007fff8703df06 libsystem_kernel.dylib`__pthread_kill + 10
        frame #1: 0x00007fff93a764ec libsystem_pthread.dylib`pthread_kill + 90
        frame #2: 0x00007fff97c056df libsystem_c.dylib`abort + 129
        frame #3: 0x00007fff97bccdd8 libsystem_c.dylib`__assert_rtn + 321
        frame #4: 0x0000000107a4e106 nvim`uv__close(fd=<unavailable>) + 102 at core.c:521
        frame #5: 0x0000000107a5307d nvim`uv__loop_close(loop=0x00007fff5847c018) + 77 at loop.c:118
        frame #6: 0x0000000107a4d149 nvim`uv_loop_close(loop=0x00007fff5847c018) + 57 at uv-common.c:626
        frame #7: 0x000000010783e5bc nvim`stream_set_blocking(fd=0, blocking=true) + 204 at stream.c:34
        frame #8: 0x000000010795d66b nvim`mch_exit(r=0) + 91 at os_unix.c:147
        frame #9: 0x00000001078d5663 nvim`command_line_scan(parmp=0x00007fff5847c760) + 1779 at main.c:787
        frame #10: 0x00000001078d4393 nvim`main(argc=2, argv=0x00007fff5847c898) + 163 at main.c:249
        frame #11: 0x00007fff8cdd65ad libdyld.dylib`start + 1
        frame #12: 0x00007fff8cdd65ad libdyld.dylib`start + 1

0 files changed, 0 insertions, 0 deletions