Describe the bug

When the path provided by the user does not exist (i.e. when the directory does not exist), the error message raised by the RAWNirx __init__ function is

RuntimeError: Expect one evt file, got 0

I think it should be something like

RuntimeError: The path you specified does not exist

since the error is not about the existence of a file, but rather about the directory's

Steps to reproduce

Create a RAWNirx instance by reading the data of a non-existent directory :

fnirs_raw_dir = 'a/path/that/doesnt/exist'

raw_intensity = mne.io.read_raw_nirx(fnirs_raw_dir,
                                         verbose=True).load_data()

Expected results

The error message tells the user the path to the specified directory does not exist.

Actual results

The error message tells the user the specified directory lacks an *.evt file (since it is the first file extension that is checked) but not that the directory does not exist: the existence of the directory is not checked, which is misleading the user.

Additional information

Platform: Windows-10-10.0.18362-SP0
Python: 3.8.3 (default, May 19 2020, 06:50:17) [MSC v.1916 64 bit (AMD64)]
Executable: C:\Users\juliend\Anaconda3\envs\mne_dev\python.exe
CPU: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel: 8 cores
Memory: 15.6 GB

mne: 0.21.dev0
numpy: 1.18.1 {blas=mkl_rt, lapack=mkl_rt}
scipy: 1.4.1
matplotlib: 3.1.3 {backend=Qt5Agg}

sklearn: 0.22.1
numba: 0.49.1
nibabel: 3.1.0
cupy: Not found
pandas: 1.0.3
dipy: 1.1.1
mayavi: 4.7.2.dev0
pyvista: 0.25.3
vtk: 9.0.0
PyQt5: 5.9.2

--

Please note that adding

if not op.exists(fname):
    raise RuntimeError('The path you specified does not exist.')

or

if not op.isdir(fname):
    raise RuntimeError('The path you specified does not exist.')

at the start of __init function of RAWNirx does not [EDIT: see this comment] lead to raising the error, if fname is a non-existent path; there must be a "good" reason for that. I'm in to correct the behaviour if you think it should be fixed.