Print a nice error of Open3D is not installed

[moratom]

No description provided.

[SzabolcsGergely]

Rather use sys.executable instead python3.
Also raise instead exit, or raise a new exception with this message

[SzabolcsGergely]

import sys

try:
    import open3d as o3d
except ImportError:
    sys.exit("Critical dependency missing: Open3D. Please install it using the command: '{} -m pip install open3d' and then rerun the script.".format(sys.executable))

[moratom]

Agree on sys.executable.

Why raise instead of an print and an exit though? If we catch exactly what the error is - isn't it cleaner to output a clean "what to do" message than what's the output of an uncatched error:

matevz@matevz-luxmmPC:~/Luxonis/Depthai/depthai-python/examples/PointCloud$ python visualize_pointcloud.py 
Open3D not installed, to be able to run the example install Open3D with: /usr/bin/python -m pip install open3d
Traceback (most recent call last):
  File "visualize_pointcloud.py", line 8, in <module>
    import open3d as o3d
ModuleNotFoundError: No module named 'open3d'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "visualize_pointcloud.py", line 12, in <module>
    raise ImportError(errorMessage)
ImportError: Open3D not installed, to be able to run the example install Open3D with: /usr/bin/python -m pip install open3d

EDIT: I didn't see the second comment, agree with the change.

[SzabolcsGergely]

Yeah, use sys.exit("message") imo