[BUG] Update BFS and SSSP to check start/source vertex for validity

[alexbarghi-nv]

Adds a check in BFS and SSSP to ensure that the start vertex is a valid vertex in the provided graph. Returns a ValueError if the start vertex is invalid. Closes #1587

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Codecov Report

Merging #2268 (6e02fd5) into branch-22.06 (38be932) will decrease coverage by 1.07%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-22.06    #2268      +/-   ##
================================================
- Coverage         70.82%   69.75%   -1.08%     
================================================
  Files               170      175       +5     
  Lines             11036    11495     +459     
================================================
+ Hits               7816     8018     +202     
- Misses             3220     3477     +257     

Impacted Files	Coverage Δ
python/cugraph/cugraph/__init__.py	100.00% <ø> (ø)
python/cugraph/cugraph/centrality/__init__.py	100.00% <ø> (ø)
...graph/cugraph/centrality/betweenness_centrality.py	89.65% <ø> (ø)
...on/cugraph/cugraph/centrality/degree_centrality.py	81.81% <ø> (ø)
...thon/cugraph/cugraph/centrality/katz_centrality.py	88.23% <ø> (-1.24%)	⬇️
python/cugraph/cugraph/community/egonet.py	97.36% <ø> (ø)
...ython/cugraph/cugraph/community/ktruss_subgraph.py	85.29% <ø> (ø)
...cugraph/cugraph/dask/centrality/katz_centrality.py	25.00% <ø> (ø)
python/cugraph/cugraph/dask/common/input_utils.py	22.13% <ø> (+0.57%)	⬆️
python/cugraph/cugraph/dask/common/mg_utils.py	30.43% <ø> (-7.07%)	⬇️
... and 108 more

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[rlratzel]

Looks good, thank you for also adding the additional tests.

[BradReesWork]

@gpucibot merge