Consider failing early for reading >100 MiB files

[craigwalton-dsit]

SandboxEnvironment.read_file() on files over 100 MiB should raise an OutputLimitExceededError error.

At present, we'll raise this error upon reading 100 MiB + 1 byte, but we could fail early by adding some check that the file is <= 100 MiB before starting the read (acknowledging that there is a race condition). This would add a bit of complexity, but maybe we'd wrap it up in a small /bin/sh script.