cli: improve configuration failure mode
cli: improve configuration failure mode

This improves the error message printed when ripgrep can't read the
file path pointed to by RIPGREP_CONFIG_PATH. Specifically, before this
change:

    $ RIPGREP_CONFIG_PATH=no_exist_path rg 'search regex'
    no_exist_path: No such file or directory (os error 2)

And now after this change:

    $ RIPGREP_CONFIG_PATH=no_exist_path rg 'search regex'
    failed to read the file specified in RIPGREP_CONFIG_PATH: no_exist_path: No such file or directory (os error 2)

In the above examples, the first failure mode looks obvious, but that's
only because RIPGREP_CONFIG_PATH is being set at the same time that we
run the command. Often, the environment variable is set elsewhere and
the error message could be confusing outside of that context.

Closes #1990

BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep