always capture exceptions by const references

For #199.

* tests/sanity/style.test: Test this.
* bin/ltlcross.cc, spot/parseaut/parseaut.yy,
tests/ltsmin/modelcheck.cc: Fix it.
10 jobs for next in 167 minutes and 12 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed arch-clang #2952

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passed arch-gcc-glibcxxdebug #2953

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passed debian-gcc-snapshot #2951

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passed debian-stable-gcc #2949

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passed debian-unstable-gcc-coverage #2950

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88.9%
passed debpkg-stable #2956

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passed debpkg-unstable #2957

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passed mingw-shared #2954

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passed mingw-static #2955

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  Publish
passed publish-unstable #2958
dput

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