Failed to process trace

```
$ $(which strace) -ff -y -ttt -qq -a1 -o /tmp/outputs/ls- ls -lah /tmp/
......
$ ls /tmp/outputs
ls-.1301978

$ tracecode parse /tmp/outputs /tmp/parsed_outputs
INFO:tracecode:Processing traces with cwd: None, input_dir: '/tmp/outputs', output_dir: '/tmp/parsed_outputs'.
INFO:tracecode:validate_traces: Found 1 traces with root pid: 1301978.
INFO:tracecode:validate_traces: Oldest pid: 1301978.
INFO:tracecode:validate_traces: Smallest pid: 1301978.
INFO:tracecode:Queuing trace of pid 1301978 for parsing. Left to do: 0
INFO:tracecode:All 1 traces queued for parsing in 0.26 seconds.
INFO:tracecode:Filtering and saving cleaned traces to '/tmp/parsed_outputs'.
INFO:tracecode:Applied filters to 1 traces in 0.00 seconds.
INFO:tracecode:Filtered 0 empty traces from '/tmp/parsed_outputs' with 1 cycles in 0.00 seconds.
INFO:tracecode:Processing completed in 0.60 seconds. All 1 traces parsed and saved to: "/tmp/parsed_outputs".
INFO:tracecode:Completed in 0.60 seconds.

$ ls /tmp/parsed_outputs
1301978.pickle
$ ls /tmp/a
$ ls /tmp/b
$ tracecode list /tmp/parsed_outputs /tmp/a/reads /tmp/b/writes
INFO:tracecode:Loading traces ...
INFO:tracecode:Building file lists ...
INFO:tracecode:Saving file lists ...
Traceback (most recent call last):
  File "tracecode/tracecode-toolkit-strace/venv/bin/tracecode", line 8, in
    sys.exit(cli())
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2606, in cli
    main(arguments)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2547, in main
    save_file_lists(input_dir, reads_file, writes_file, debug=debug)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2255, in save_file_lists
    save_sorted(reads_file, all_reads)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2317, in save_sorted
    with open(file_out) as fo:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/a/reads'
$ tracecode list /tmp/parsed_outputs /tmp/a /tmp/b
INFO:tracecode:Loading traces ...
INFO:tracecode:Building file lists ...
INFO:tracecode:Saving file lists ...
Traceback (most recent call last):
  File "tracecode/tracecode-toolkit-strace/venv/bin/tracecode", line 8, in
    sys.exit(cli())
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2606, in cli
    main(arguments)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2547, in main
    save_file_lists(input_dir, reads_file, writes_file, debug=debug)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2255, in save_file_lists
    save_sorted(reads_file, all_reads)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2317, in save_sorted
    with open(file_out) as fo:
IsADirectoryError: [Errno 21] Is a directory: '/tmp/a'
$ touch /tmp/a/reads
$ tracecode list /tmp/parsed_outputs /tmp/a /tmp/b
INFO:tracecode:Loading traces ...
INFO:tracecode:Building file lists ...
INFO:tracecode:Saving file lists ...
Traceback (most recent call last):
  File "tracecode/tracecode-toolkit-strace/venv/bin/tracecode", line 8, in
    sys.exit(cli())
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2606, in cli
    main(arguments)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2547, in main
    save_file_lists(input_dir, reads_file, writes_file, debug=debug)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2255, in save_file_lists
    save_sorted(reads_file, all_reads)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2317, in save_sorted
    with open(file_out) as fo:
IsADirectoryError: [Errno 21] Is a directory: '/tmp/a'
$ tracecode list /tmp/parsed_outputs /tmp/a/reads /tmp/b
INFO:tracecode:Loading traces ...
INFO:tracecode:Building file lists ...
INFO:tracecode:Saving file lists ...
Traceback (most recent call last):
  File "tracecode/tracecode-toolkit-strace/venv/bin/tracecode", line 8, in
    sys.exit(cli())
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2606, in cli
    main(arguments)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2547, in main
    save_file_lists(input_dir, reads_file, writes_file, debug=debug)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2255, in save_file_lists
    save_sorted(reads_file, all_reads)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2318, in save_sorted
    fo.write("\n".join(sorted(seq)))
io.UnsupportedOperation: not writable
$ tracecode list /tmp/parsed_outputs /tmp/a/reads /tmp/b
$ touch /tmp/a/writes
$ tracecode list /tmp/parsed_outputs /tmp/a/reads /tmp/b/writes
INFO:tracecode:Loading traces ...
INFO:tracecode:Building file lists ...
INFO:tracecode:Saving file lists ...
Traceback (most recent call last):
  File "tracecode/tracecode-toolkit-strace/venv/bin/tracecode", line 8, in
    sys.exit(cli())
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2606, in cli
    main(arguments)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2547, in main
    save_file_lists(input_dir, reads_file, writes_file, debug=debug)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2255, in save_file_lists
    save_sorted(reads_file, all_reads)
  File "tracecode/tracecode-toolkit-strace/src/tracecode/tracecode.py", line 2318, in save_sorted
    fo.write("\n".join(sorted(seq)))
io.UnsupportedOperation: not writable
```