Better error message for divergence with no dt_min

[RemDelaporteMathurin]

Running this on FESTIM 1.3.1 produces:

import festim as F
import numpy as np

my_model = F.Simulation(log_level=40)

tungsten = F.Material(id=1, D_0=4.10e-7, E_D=0.39)
my_model.materials = tungsten


my_model.mesh = F.MeshFromVertices(vertices=np.linspace(0, 1, 10))
my_model.T = 400


my_model.boundary_conditions = [
    F.DirichletBC(surfaces=[1, 2], value=1e20, field="solute")
]

my_model.dt = F.Stepsize(0.001, stepsize_change_ratio=1.1)


my_model.settings = F.Settings(
    absolute_tolerance=1e-10,
    relative_tolerance=1e-10,
    final_time=100,
)

my_model.initialise()
my_model.run()

Defining initial values
Defining variational problem
Defining source terms
Defining boundary conditions
Time stepping...
Traceback (most recent call last):time so far: 0.0 s
  File "/home/remidm/FESTIM/mwe.py", line 28, in <module>
    my_model.run()
  File "/home/remidm/FESTIM/festim/generic_simulation.py", line 422, in run
    self.run_transient()
  File "/home/remidm/FESTIM/festim/generic_simulation.py", line 445, in run_transient
    self.iterate()
  File "/home/remidm/FESTIM/festim/generic_simulation.py", line 473, in iterate
    self.h_transport_problem.update(self.t, self.dt)
  File "/home/remidm/FESTIM/festim/h_transport_problem.py", line 338, in update
    dt.adapt(t, nb_it, converged)
  File "/home/remidm/FESTIM/festim/stepsize.py", line 100, in adapt
    if float(self.value) < dt_min:
       ^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: '<' not supported between instances of 'float' and 'NoneType'

I've recently noticed that this is a situation users often encounter and we should provide a more explanatory error message.