fix bug in callbacks

[valentinsulzer]

Description

Fix a bug in callbacks for the Simulation class that was displaying the wrong error for experiments.

I keep fixing this in different branches, so hopefully doing it in its own branch will get in merged quicker.

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[brosaplanella]

Looks good, thanks! Add an entry to CHANGELOG before merging.

[codecov]

Codecov Report

Merging #2213 (cb98ea3) into develop (59c99b2) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #2213      +/-   ##
===========================================
- Coverage    99.36%   99.36%   -0.01%     
===========================================
  Files          355      355              
  Lines        19514    19511       -3     
===========================================
- Hits         19390    19387       -3     
  Misses         124      124              

Impacted Files	Coverage Δ
pybamm/callbacks.py	100.00% <100.00%> (ø)
pybamm/simulation.py	98.83% <100.00%> (+<0.01%)	⬆️
pybamm/solvers/casadi_solver.py	98.25% <100.00%> (-0.01%)	⬇️

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