Propagate error code in case of client reset errors in the callback

[nicola-cab]

What, How & Why?

When there is an error (an exception) thrown inside user code callbacks, we usually provide some hooks for the SDKs in order to register a message for the exception just thrown and retrieve it back once core stops executing the callback.
This was done for realm_config_set_migration_function, realm_config_set_data_initialization_function and realm_config_set_should_compact_on_launch_function but not for the 2 client reset callbacks.
In this case, the code is more completed because it is embedded inside the client reset logic and the session machinery. I have drafted a possible implementation in order to propagate possible errors happening inside user code callbacks while client reset is running.

Fixes: #7098

☑️ ToDos

• 📝 Changelog update
• 🚦 Tests (or not relevant)
• C-API, if public C++ API changed
• bindgen/spec.yml, if public C++ API changed

[coveralls-official]

Pull Request Test Coverage Report for Build nicola.cabiddu_1195

• 94 of 97 (96.91%) changed or added relevant lines in 5 files are covered.
• 173 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.05%) to 91.704%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/realm/object-store/c_api/config.cpp	3	4	75.0%
src/realm/object-store/c_api/error.cpp	6	8	75.0%

Files with Coverage Reduction	New Missed Lines	%
src/realm/index_string.cpp	1	87.78%
src/realm/sort_descriptor.cpp	3	93.7%
src/realm/sync/client.cpp	3	91.44%
src/realm/sync/network/network.cpp	3	89.66%
src/realm/sync/noinst/client_impl_base.cpp	3	85.37%
src/realm/table.cpp	3	90.63%
src/realm/object-store/sync/sync_manager.cpp	4	86.31%
src/realm/util/assert.hpp	4	87.1%
src/realm/sync/instruction_applier.cpp	8	70.26%
test/fuzz_group.cpp	23	56.53%

Totals
Change from base Build 1897:	-0.05%
Covered Lines:	232143
Relevant Lines:	253145

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💛 - Coveralls