Raise the exception when LoadImage has a reader specified but it is not installed #7437
Description
Describe the bug
Running LoadImage(reader="ITKReader") without itk installed will just warn that itk is not installed and will proceed with using PILReader instead.
MONAI/monai/transforms/io/array.py
Line 185 in 33afaef
Expected behavior
If the specified reader is not installed, an exception should be raised.