chore(crypto): Add consistency check on device when loading account

[BillCarsonFr]

Add a consistency check when loading an account from a store.

When creating an olm machine for an empty store, the own device is explicitely saved before the account (and it is done on purpose before the initial keys/query response):

matrix-rust-sdk/crates/matrix-sdk-crypto/src/machine/mod.rs

Lines 328 to 340 in 210c574

	let device = DeviceData::from_account(&account);
	// We just created this device from our own Olm `Account`. Since we are the
	// owners of the private keys of this device we can safely mark
	// the device as verified.
	device.set_trust_state(LocalTrust::Verified);
	let changes = Changes {
	devices: DeviceChanges { new: vec![device], ..Default::default() },
	..Default::default()
	};
	store.save_changes(changes).await?;
	store.save_pending_changes(PendingChanges { account: Some(account) }).await?;

However when loading an existing account we don't check for the existence of this own device.
There are other part in the code when we expect the own device to always exists:

matrix-rust-sdk/crates/matrix-sdk-sqlite/src/crypto_store.rs

Line 1205 in 210c574

.expect("We should be able to find our own device."))

Let's do the consistency check as soon as possible to avoid future strange states.

• Public API changes documented in changelogs (optional)

Signed-off-by:

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.41%. Comparing base (210c574) to head (c3d099b).

Files with missing lines	Patch %	Lines
crates/matrix-sdk-crypto/src/machine/mod.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4564      +/-   ##
==========================================
+ Coverage   85.40%   85.41%   +0.01%     
==========================================
  Files         285      285              
  Lines       32218    32221       +3     
==========================================
+ Hits        27515    27521       +6     
+ Misses       4703     4700       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[richvdh]

I'm not enthusiastic about adding this here as yet another error code that any API can return: error handling is really hard when you call matrix-sdk-crypto APIs because there are thousands of possible error cases.

Maybe we can specialise the result from with_store?

[BillCarsonFr]

Yes I was not sure about this one. What do you mean by "specialise" the result?

[richvdh]

make it return a different error type in its Result, which more closely reflects the actual possible failure modes of that particular method.

[bnjbvr]

Marking request changes for Rich's comment, which I agree with.