Fix error message upon misalignment in PadDynamicalDecoupling

[Cryoris]

Summary

If a delay is misaligned and following a DAGInNode (or preceding a DAGOutNode), the error message in PadDynamicalDecoupling previously failed, as it was trying to access non-existing attributes. This is because, unlike DAGOpNode, the in and out nodes do not have a name or qargs attribute.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[jakelishman]

Just for my understanding: is the expectation that the (correct) error message should only show if there's been a bug in the prior PadDelay pass, or are we expecting users to feed in manually scheduled circuits?

[coveralls]

Pull Request Test Coverage Report for Build 10384311178

Details

• 4 of 5 (80.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 89.703%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/transpiler/passes/scheduling/padding/dynamical_decoupling.py	4	5	80.0%

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	91.73%

Totals
Change from base Build 10374261794:	0.03%
Covered Lines:	67428
Relevant Lines:	75168

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💛 - Coveralls

[Cryoris]

As I understand it, this error message should only come up if users put manual delays into the circuit and either didn't run ConstrainedReschedule or put barriers, such that the reschedule doesn't merge delays into multiples of the alignment.