#803 make visualise fail only raise logger error

[valentinsulzer]

Description

Make visualise raise a logger error only if graphviz is not installed (so that graphviz is an optional dependency)
Fixes #803

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

[codecov]

Codecov Report

Merging #810 into master will decrease coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #810      +/-   ##
==========================================
- Coverage   98.14%   98.12%   -0.02%     
==========================================
  Files         179      179              
  Lines        9919     9922       +3     
==========================================
+ Hits         9735     9736       +1     
- Misses        184      186       +2

Impacted Files	Coverage Δ
pybamm/expression_tree/symbol.py	96.99% <50%> (-0.73%)	⬇️

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