Improve error msg for plot coils when non-coil object is passed in

[dpanici]

Inspired by #1195

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.45%. Comparing base (c7495d4) to head (427e046).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1198      +/-   ##
==========================================
- Coverage   95.46%   95.45%   -0.01%     
==========================================
  Files          87       87              
  Lines       22265    22266       +1     
==========================================
  Hits        21255    21255              
- Misses       1010     1011       +1     

Files	Coverage Δ
desc/plotting.py	95.81% <100.00%> (+<0.01%)	⬆️

... and 2 files with indirect coverage changes

[github-actions]

|             benchmark_name             |         dt(%)          |         dt(s)          |        t_new(s)        |        t_old(s)        | 
| -------------------------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- |
 test_build_transform_fft_lowres         |     -0.80 +/- 8.51     | -4.21e-03 +/- 4.49e-02 |  5.23e-01 +/- 4.3e-02  |  5.28e-01 +/- 1.4e-02  |
 test_build_transform_fft_midres         |     +0.83 +/- 4.24     | +5.05e-03 +/- 2.57e-02 |  6.11e-01 +/- 2.2e-02  |  6.06e-01 +/- 1.4e-02  |
 test_build_transform_fft_highres        |     +3.14 +/- 3.25     | +3.15e-02 +/- 3.26e-02 |  1.03e+00 +/- 2.7e-02  |  1.00e+00 +/- 1.8e-02  |
 test_equilibrium_init_lowres            |     +3.84 +/- 4.68     | +1.50e-01 +/- 1.82e-01 |  4.04e+00 +/- 1.4e-01  |  3.89e+00 +/- 1.2e-01  |
 test_equilibrium_init_medres            |     +5.39 +/- 4.78     | +2.34e-01 +/- 2.07e-01 |  4.57e+00 +/- 1.6e-01  |  4.34e+00 +/- 1.4e-01  |
 test_equilibrium_init_highres           |     +3.82 +/- 1.96     | +2.18e-01 +/- 1.12e-01 |  5.92e+00 +/- 6.8e-02  |  5.70e+00 +/- 8.9e-02  |
 test_objective_compile_dshape_current   |     +2.38 +/- 1.22     | +9.43e-02 +/- 4.85e-02 |  4.05e+00 +/- 3.6e-02  |  3.96e+00 +/- 3.3e-02  |
 test_objective_compile_atf              |     +0.75 +/- 3.31     | +6.38e-02 +/- 2.82e-01 |  8.56e+00 +/- 1.7e-01  |  8.49e+00 +/- 2.2e-01  |
 test_objective_compute_dshape_current   |     +2.65 +/- 4.26     | +3.37e-05 +/- 5.41e-05 |  1.31e-03 +/- 3.1e-05  |  1.27e-03 +/- 4.5e-05  |
 test_objective_compute_atf              |     +0.95 +/- 5.43     | +4.18e-05 +/- 2.37e-04 |  4.42e-03 +/- 2.0e-04  |  4.38e-03 +/- 1.3e-04  |
 test_objective_jac_dshape_current       |     +4.67 +/- 10.02    | +1.79e-03 +/- 3.84e-03 |  4.01e-02 +/- 2.8e-03  |  3.83e-02 +/- 2.7e-03  |
 test_objective_jac_atf                  |     +1.11 +/- 3.56     | +2.07e-02 +/- 6.62e-02 |  1.88e+00 +/- 5.6e-02  |  1.86e+00 +/- 3.6e-02  |
 test_perturb_1                          |     +2.28 +/- 1.79     | +3.27e-01 +/- 2.57e-01 |  1.47e+01 +/- 1.4e-01  |  1.43e+01 +/- 2.1e-01  |
 test_perturb_2                          |     +1.53 +/- 1.60     | +2.98e-01 +/- 3.12e-01 |  1.98e+01 +/- 1.2e-01  |  1.95e+01 +/- 2.9e-01  |
 test_proximal_jac_atf                   |     +0.98 +/- 1.01     | +7.18e-02 +/- 7.43e-02 |  7.39e+00 +/- 4.3e-02  |  7.32e+00 +/- 6.1e-02  |
 test_proximal_freeb_compute             |     -1.04 +/- 0.97     | -1.89e-03 +/- 1.76e-03 |  1.80e-01 +/- 1.0e-03  |  1.82e-01 +/- 1.4e-03  |
 test_proximal_freeb_jac                 |     +1.46 +/- 1.79     | +1.08e-01 +/- 1.33e-01 |  7.54e+00 +/- 1.2e-01  |  7.43e+00 +/- 5.7e-02  |
 test_solve_fixed_iter                   |     +1.98 +/- 5.98     | +3.63e-01 +/- 1.10e+00 |  1.87e+01 +/- 8.4e-01  |  1.83e+01 +/- 7.0e-01  |