sequencer-relayer: add utility function for asserting error variants

[ethanoroshiba]

We should add a utility function assert_error_variant to avoid manual matches throughout both these files:

astria/crates/astria-sequencer-relayer/src/relayer/celestia_client/celestia_keys.rs

Lines 127 to 131 in 70046bd

	// allow: `assert!(matches!(..))` provides poor feedback on failure.
	#[allow(clippy::manual_assert)]
	if !matches!(error, Error::ReadFile(_)) {
	panic!("expected an error variant `Error::ReadFile`, got {error:?}");
	}

astria/crates/astria-sequencer-relayer/src/relayer/celestia_client/tests.rs

Lines 58 to 63 in 70046bd

	// allow: `assert!(matches!(..))` provides poor feedback on failure.
	#[allow(clippy::manual_assert)]
	if !matches!(error, TrySubmitError::BlobTooLarge { byte_count } if byte_count == u32::MAX as usize + 1)
	{
	panic!("expected `Error::BlobTooLarge` with byte_count == u32::MAX + 1, got {error:?}");
	}

┆Issue Number: ENG-873