Fix SEVIRI native reader failing when missing main header

[ameraner]

This PR fixes a bug introduced by #2198 (cc @simonrp84 ) that causes the SEVIRI reader to fail for files that do not have a 15_MAIN_PRODUCT_HEADER -this header is read, optionally, here:

satpy/satpy/readers/seviri_l1b_native_hdr.py

Lines 84 to 86 in f44473f

	if with_archive_header:
	record += [
	('15_MAIN_PRODUCT_HEADER', L15MainProductHeaderRecord().get()),

Files that do not contain the archive header were missing the 15_MAIN_PRODUCT_HEADER, which was causing a KeyError.

After this PR, an info log message is issued specifying that the quality check could not be performed in case of the missing header.

• Tests added

[codecov]

Codecov Report

Merging #2516 (7432312) into main (9415ad9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2516   +/-   ##
=======================================
  Coverage   94.85%   94.85%           
=======================================
  Files         337      337           
  Lines       49624    49643   +19     
=======================================
+ Hits        47069    47088   +19     
  Misses       2555     2555           

Flag	Coverage Δ
behaviourtests	4.41% <0.00%> (-0.01%)	⬇️
unittests	95.47% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_native.py	86.82% <100.00%> (+0.06%)	⬆️
satpy/tests/reader_tests/test_seviri_l1b_native.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[coveralls]

Pull Request Test Coverage Report for Build 5336249345

• 9 of 9 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 95.421%

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Totals
Change from base Build 5323400933:	0.001%
Covered Lines:	47201
Relevant Lines:	49466

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💛 - Coveralls

[mraspaud]

LGTM