dag_drawer should check the existence of filename extension

Information

• Qiskit Terra version: 0.10.0.dev0+831d942
• Python version: 3.7
• Operating system: Mac

What is the current behavior?

If a filename without extension is passed to the function dag_drawer, this line reports two errors:

nxpd.pydot.InvocationException: Program terminated with status: 1. stderr follows: Format: "XXXXX" not recognized. Use one of: ......
During handling of the above exception, another exception occurred:
qiskit.visualization.exceptions.VisualizationError: 'dag_drawer requires GraphViz installed in the system. Check https://www.graphviz.org/download/ for details on how to install GraphViz in your system.'

This is confusing because the second error thrown by Qiskit is not the cause of the problem.

Steps to reproduce the problem

Try dag_drawer(dag, filename='abc')

What is the expected behavior?

Make the error catching better.

Suggested solutions

We could either catch this error by reading and filtering the error message, or we could check the existence of the filename’s extension, and provide a default one.

[iuliazidaru]

@javabster Please assign the issue to me.

[1ucian0]

I can have a look!