Use rust-native error for UnitarySynthesis

[raynelfss]

The following commits modify UnitarySynthesis to use rust-native errors throughout its pipeline. We implement the UnitarySynthesisError enumeration which derives from:

• DAGError;
• PyErr for XXDecomposer and python gates purposes only;
• QSDError from Synthesis;
• and implements two variants: NoPreferredDirection and SynthesisFidelityOutOfRange used with decomposers.

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code:

[qiskit-bot]

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

• @Qiskit/terra-core

[coveralls]

Coverage Report for CI Build 26655587578

Warning

Build has drifted: This PR's base is out of sync with its target branch, so coverage data may include unrelated changes.
Quick fix: rebase this PR. Learn more →

Coverage decreased (-0.007%) to 87.551%

Details

• Coverage decreased (-0.007%) from the base build.
• Patch coverage: 5 uncovered changes across 1 file (28 of 33 lines covered, 84.85%).
• 28 coverage regressions across 5 files.

Uncovered Changes

File	Changed	Covered	%
crates/transpiler/src/passes/unitary_synthesis/mod.rs	24	19	79.17%
Total (2 files)	33	28	84.85%

Coverage Regressions

28 previously-covered lines in 5 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/qasm2/src/parse.rs	12	97.15%
crates/circuit/src/parameter/symbol_expr.rs	8	73.93%
crates/qasm2/src/lex.rs	6	91.77%
crates/circuit/src/parameter/parameter_expression.rs	1	91.04%
crates/transpiler/src/passes/unitary_synthesis/mod.rs	1	96.55%

---

Coverage Stats

Relevant Lines:	123934
Covered Lines:	108505
Line Coverage:	87.55%
Coverage Strength:	963978.36 hits per line

---

💛 - Coveralls

[gadial]

LGTM. One question about which errors are raised (which is actually a legacy of the old code).

[gadial]

Why are we using QiskitError for one and PyValueError for the other?

[raynelfss]

This is how the original error was.

This is what it looked like for NoPreferredDirection:

qiskit/crates/transpiler/src/passes/unitary_synthesis/decomposers.rs

Lines 562 to 568 in 5294c2a

	return Err(QiskitError::new_err(format!(
	concat!(
	"No preferred direction of gate on qubits {:?} ",
	"could be determined from coupling map or gate lengths / gate errors."
	),
	qubits
	)));

This is for SynthesisFidelityOutOfRange

• here:

  qiskit/crates/transpiler/src/passes/unitary_synthesis/decomposers.rs

  Lines 608 to 612 in 5294c2a

  	let fidelity = config.approximation.synthesis_fidelity(0.0).map_err(|e| {
  	PyValueError::new_err(format!(
  	"requested synthesis fidelity is out of range: {e}"
  	))
  	})?;

• and here:

  qiskit/crates/transpiler/src/passes/unitary_synthesis/decomposers.rs

  Lines 699 to 706 in 5294c2a

  	let fidelity = config
  	.approximation
  	.synthesis_fidelity(candidate.error)
  	.map_err(|e| {
  	PyValueError::new_err(format!(
  	"requested synthesis fidelity is out of range: {e}"
  	))
  	})?;