Ensure that errors have as much context as possible

[hgoldstein95]

Go through and add relevant variables and parameters as context to errors, especially in commands other than serve.

The basic procedure is to find places where we use

ErrorKind::SomeKindOfError

and include variables as attachments, e.g.

ErrorKind::SomeKindOfError.attach("varname", var_value)

See error.rs for more information, and reach out to @hgoldstein95 if you need help.