We previously used to throw an error denoting where in the configuration the attribute was missing or invalid.
Considering that organization can be now be omitted from the configuration, our previous error message will be
improperly formatted. This commit also updates the message to mention `TF_ORGANIZATION` as a valid substitute if
organization is missing or invalid in the configuration.
Explicit version strings are actually also version constraints! And the special
comparisons we were doing to allow a range of compatible versions can also be
expressed as version constraints.
Bonus: also simplify the way we handle version check errors, by composing the
messages inline and only extracting the repetitive parts into a function.
