Fix panic error at shortest paths

[JPena-code]

In #1117 is presented the presence of an uncaught pyo3.PanicException when the index of the source node is no out of bound in the graph. With this naive approach i try fix this issue raising a more understandable IndexError.

• [x] I ran rustfmt locally
• [x] I have added the tests to cover my changes.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.

[coveralls]

Pull Request Test Coverage Report for Build 8207084623

Details

• 57 of 57 (100.0%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 96.446%

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Files with Coverage Reduction	New Missed Lines	%
src/shortest_path/all_pairs_bellman_ford.rs	6	95.53%
<!--	Total:	6		-->

Totals
Change from base Build 8147160812:	-0.02%
Covered Lines:	17151
Relevant Lines:	17783

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💛 - Coveralls