Rework the error message that tells users to list the tasks with rake --tasks

[colby-swandale]

This PR is reworking the error message when a given task does not exist.

$ rake foo
rake aborted!
Don't know how to build task 'foo' (see --tasks)
/Users/colby/.gem/ruby/2.5.1/gems/rake-12.3.1/exe/rake:27:in `<top (required)>'
(See full trace by running task with --trace)

There is a good opportunity here to reword the (see --tasks) into something more meaningful and direct.

Users may not be aware of the rake --tasks option, so this PR adjusts the error message to instead be direct in saying that you can see a list of available tasks using this command.

I've turned it into this, which i think provides better communication to users:

Don't know how to build task 'foo' (See the list of available tasks with `rake --tasks`)

[coveralls]

Coverage remained the same at 97.751% when pulling 0086b4a on colby-swandale:rework-invalid-task-error-message into 8e1ea2d on ruby:master.

[coveralls]

Coverage remained the same at 97.751% when pulling 0086b4a on colby-swandale:rework-invalid-task-error-message into 8e1ea2d on ruby:master.

[coveralls]

Coverage remained the same at 97.751% when pulling b6521cf on colby-swandale:rework-invalid-task-error-message into 8e1ea2d on ruby:master.