Add "no size marker detected" fatal error message to pcv.report_size_marker ?

[HaleySchuhl]

Is your feature request related to a problem? Please describe.
We generally recommend that color cards be utilized as size markers for scaling pixel measurements, but report size marker is still an option. It's a helpful wrapper for cases where someone doesn't have a color card, which is true of microscopy images for example. If no size marker object is detected under the current parameters, a rather unhelpful error message is triggered in the middle of the function.

Describe the solution you'd like
I propose we detect the lack of marker_contour or roi_mask upstream of this step in the function, and give an informative error message. Also maybe plot marker_bin overlaid on the input image as a debug to help adjust threshold up or down.

Describe alternatives you've considered
Warning message and add "size marker found" observation/return? Continues on with analysis workflow to extract data but those datapoints are unscalable later on and could get filtered out.

Additional context

----> 5 image = pcv.report_size_marker_area(img=img, roi=roi, marker='detect', 
      6                                     objcolor='light', thresh_channel='s', 
      7                                     thresh=120)

File [~/Documents/GitHub/plantcv/build/__editable__.plantcv-3.6.dev68](http://localhost:8889/lab/tree/Documents/Projects/roi/~/Documents/GitHub/plantcv/build/__editable__.plantcv-3.6.dev68)+g9e5e7b8c-py3-none-any[/plantcv/plantcv/report_size_marker_area.py:101](http://localhost:8889/plantcv/plantcv/report_size_marker_area.py#line=100), in report_size_marker_area(img, roi, marker, objcolor, thresh_channel, thresh, label)

     98 marker_area = m['m00']
    100 # Fit a bounding ellipse to the marker
--> 101 _, axes, _ = cv2.fitEllipse(marker_contour)
    102 major_axis = np.argmax(axes)
    103 minor_axis = 1 - major_axis

error: OpenCV(4.10.0) [/Users/runner/miniforge3/conda-bld/libopencv_1717728554055/work/modules/imgproc/src/shapedescr.cpp:355](http://localhost:8889/Users/runner/miniforge3/conda-bld/libopencv_1717728554055/work/modules/imgproc/src/shapedescr.cpp#line=354): error: (-215:Assertion failed) n >= 0 && (depth == CV_32F || depth == CV_32S) in function 'fitEllipseNoDirect'