hledger silently falls back to default file path if LEDGER_FILE does not exist

[simonmichael]

If LEDGER_FILE is set, but the specified file does not exist, hledger falls back to using the default journal path, eg ~/.hledger.journal. If that file exists, it will run without error, eg hledger files will show that default path. This can be confusing, especially since docs have been recommending hledger files for testing the config (hledger setup is better for this).

I have rewritten the docs at https://hledger.org/dev/hledger.html#setting-ledger_file.

And I think maybe we should report an error when LEDGER_FILE is set to a non-existent path ?