Remove all sys.exit(1) from within the code

[NOhs]

Pulling the plug just because something unexpected happend is not a calm response and makes error tracking/debugging tough. It also makes it impossible to include that code in anything else knowing that it could just exit. We should replace the occurrences of sys.exit(...) with proper exceptions which we should deal with in the main function.

[GPMueller]

Since d0a3090, only two instances remain:

clang-build/clang_build/clang_build.py

Lines 84 to 101 in d0a3090

	def _find_clang(logger):
	clangpp = _which('clang++')
	clang_ar = _which('llvm-ar')
	if clangpp:
	llvm_root = _Path(clangpp).parents[0]
	else:
	logger.error('Couldn\'t find clang++ executable')
	sys.exit(1)
	if not clang_ar:
	logger.error('Couldn\'t find llvm-ar executable')
	sys.exit(1)
	logger.info(f'llvm root directory: {llvm_root}')
	logger.info(f'clang++ executable: {clangpp}')
	logger.info(f'llvm-ar executable: {clang_ar}')
	logger.info(f'Newest supported C++ dialect: {_get_max_supported_compiler_dialect(clangpp)}')
	return clangpp, clang_ar

[GPMueller]

This was finished by the merge of PR #53 (e0cd95e).