Avoid segmentation fault with NOTMUCH_CONFIG=/new/config/file notmuch setup

Avoid segmentation fault with NOTMUCH_CONFIG=/new/config/file notmuch setup

Since commit f41a35e2923b0b3ef2f66e91cd30e20b4c1a336a running a command
such as:

	NOTMUCH_CONFIG=/new/config/file notmuch setup

would result in a segmentation fault.

The purpose of that commit was to ensure that an attempt to manipulate
a non-standard database would not inadvertently manipulate the default
database only due to a typo in the NOTMUCH_CONFIG environment
variable. That is, a command like:

	NOTMUCH_CONFIG=mistyped-config-filename notmuch tag -new tag:new

shouldn't modify the database at ${HOME}/mail, but should instead
simply report that the mistype configuration filename does not exist.

We fix both cases simultaneously by reporting the error message
whenever the function calling notmuch_config_open is not explicitly
prepared for a default configuration file.