Bug report: Incorrect error message raised when non-existent turbine specified

[misi9170]

LOW PRIORITY

check_turbine_type() in farm.py raises a confusing error when a turbine model is specified that is not available in the library. The error raised is ValueError: The turbine type: <non-existent_turbine> exists in both the internal and external turbine library., whereas I would expect an error along the lines of ValueError: The turbine type: <non-existent_turbine> does not exist in the turbine library.

Note that this bug does not affect running code, and can be treated as low priority.

Steps to produce

1. Change the turbine_type field of examples/inputs/gch.yaml from nrel_5MW to bad_turbine_type.
2. Run examples/01_opening_floris_computing_power.py