import GPUNotAvailableError to error.py

[melodyyzh]

Description

Imported GPUNotAvailableError in error.py.

Will remove util.GPNotavailableError in a branch off trunk-major after this branch is merged

Motivation and context

util should hold utility methods and error should hold error classes.

Resolves #1649

How has this been tested?

Change log

Added

* ``hoomd.error.GPUNotAvailableError``
  (`#1694 <https://github.com/glotzerlab/hoomd-blue/pull/1694>`__).

Deprecated:

* ``hoomd.util.GPUNotAvailableError``
  (`#1694 <https://github.com/glotzerlab/hoomd-blue/pull/1694>`__).

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.

[tommy-waltmann]

In the referenced issue (#1649), it states that this PR should also "Note in the documentation that util.GPUNotAvailableError is deprecated and will move to error."

[joaander]

To complete the move in a backward compatible way:

1. Move _NoGPU from util.py to error.py.
2. Adjust all uses of _NoGPU to import it from error.
3. Add GPUNotAvailableError to module-hoomd-errors.rst so it appears in the documentation.
4. Remove the definition of GPUNotAvailableError from util.py and replace it with an import of hoomd.error.GPUNotAvailableError to preserve backward compatibility.
5. Note the deprecation in deprecated.rst.

Merge trunk-minor into move_GPUNotAvailableError first to avoid creating conflicts in deprecated.rst.

[joaander]

Thanks!