Unify OpenQASM 2 exceptions

[jakelishman]

Summary

Since gh-9784 added a new qiskit.qasm2 package and suitable exceptions for both import and export of OpenQASM 2 programs, this commit now unifies the exception handling through the rest of the library to use these exceptions.

To avoid breaks in compatibility, the old qiskit.qasm.QasmError is now a re-export of qiskit.qasm2.QASM2Error. This is the base error of both the exporter and importers, despite the old documentation of QasmError claiming to be specifically for parser errors. In practice, the exporter also emitted QasmError, so having that be QASM2Error allows people catching that error to get the same behaviour in these new forms. The actual exporter and importer are updated to emit the precise subclasses.

Details and comments

Probably this will clash a little with #9955 (and this PR technically probably wants to merge before that one) and potentially #9953. I'll fix any merge conflicts as they arise.

This PR is preparing for a later total deprecation and removal of the qiskit.qasm namespace, likely starting in Terra 0.26, since we've begun the process of moving to qiskit.qasm2 and qiskit.qasm3 as explicitly versioned packages, to avoid confusion in the future.

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[coveralls]

Pull Request Test Coverage Report for Build 5548013092

• 6 of 6 (100.0%) changed or added relevant lines in 3 files are covered.
• 12 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.006%) to 86.007%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	93.76%
qiskit/extensions/quantum_initializer/squ.py	2	80.0%
crates/qasm2/src/lex.rs	3	90.89%
crates/qasm2/src/parse.rs	6	96.65%

Totals
Change from base Build 5547947273:	0.006%
Covered Lines:	71891
Relevant Lines:	83587

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💛 - Coveralls

[jakelishman]

The problems in the QPY backwards compatibility tests are likely because it uses a slightly unusual ordering and import structure for its objects, and there are nasty circular dependencies between qiskit.qasm and qiskit.circuit. Once those circular imports are solved, it should not be an issue for the import structure in the QPY backwards compatibility tests if there's a temporary dependency between qiskit.qasm and qiskit.qasm2, since both will be fully at the same level of the hierarchy.

See #9959 for the fix on that.

[jlapeyre]

Arg. I made every effort to ensure no conflicts... Anyway, a blank line and an unwanted import

[jakelishman]

John: what are you trying to do to this PR? It was already up-to-date and rebased completely on top of main before your force push.

[jlapeyre]

I convinced myself that it was not correct (because it was rebased on most recent main, and things disappeared). But, I agree that your rebase on main was already correct. I also think that what I force pushed was also the same as what you did.

[jlapeyre]

I fixed or avoided the two errors above here: jakelishman#14

[jakelishman]

Ok, I've reverted it back to the previous state and put in the correct test fix - the issue is that a change in some of the imports caused qiskit.qasm2 to be loaded when previously it wasn't, which caused two "new" Gate subclasses to be picked up by an automated discovery in a place that they shouldn't have been.

[jlapeyre]

I was just comparing your last branch to mine. I think I rebased on top of main that was less up to date than yours. So I agree reverting was correct.

picked up by an automated discovery in a place that they shouldn't have been.

I assumed it was intended. I may have tried harder with DefinedGate. But I ran into an opaque compiled extension when I wanted to look at OpCode. Of course, I could have looked into that source.

[jlapeyre]

This looks ready