Handle reference to non-existent concept identity in SDMX-ML 2.1

[khaeru]

Closes #205.

PR checklist

• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.94%. Comparing base (240dd89) to head (87893de).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
- Coverage   98.90%   97.94%   -0.97%     
==========================================
  Files          97       97              
  Lines        8051     8073      +22     
==========================================
- Hits         7963     7907      -56     
- Misses         88      166      +78     

Files with missing lines	Coverage Δ
sdmx/model/common.py	99.90% <ø> (ø)
sdmx/reader/xml/v21.py	99.18% <100.00%> (-0.13%)	⬇️
sdmx/testing/__init__.py	99.35% <ø> (ø)
sdmx/tests/reader/test_reader_xml_v21.py	100.00% <100.00%> (ø)
sdmx/tests/test_sources.py	93.23% <ø> (-5.34%)	⬇️

... and 17 files with indirect coverage changes

[khaeru]

FYI @nicolas-graves —the added log messages and annotations are tested here:

sdmx/sdmx/tests/reader/test_reader_xml_v21.py

Lines 257 to 279 in 1ea3364

	def test_gh_205(caplog, specimen) -> None:
	"""Test of https://github.com/khaeru/sdmx/issues/205."""
	with specimen("INSEE/gh-205.xml") as f:
	msg = sdmx.read_sdmx(f)
	# Messages were logged
	msg_template = "Could not resolve {cls}.concept_identity reference to ConceptScheme=FR1:CONCEPTS_INSEE(1.0) → Concept={id}"
	m1 = msg_template.format(cls="TimeDimension", id="TIME_PERIOD")
	m2 = msg_template.format(cls="PrimaryMeasure", id="OBS_VALUE")
	assert m1 in caplog.messages
	assert m2 in caplog.messages
	# Access the parsed DSD
	dsd = msg.structure["CNA-2014-PIB"]
	# Components have annotations with expected ID and text
	for component, text in (
	(dsd.dimensions.get("TIME_PERIOD"), m1),
	(dsd.measures.get("OBS_VALUE"), m2),
	):
	a = component.annotations[0]
	assert "sdmx.reader.xml.v21-parse-error" == a.id
	assert text == str(a.text)