Add exception for Errors during parsing some lines

There are occasions where the parsing of a line could fail. This
exceptions needs the failed line as mandatory first argument and takes a
message as optional second argument.
Add function to split line(s) into defined columns

The reference file is structured into three or four columns, which can
have their own linebreakes. It is easier to parse each column
seperately, therefor this function reassembles the columns. However:
currently only column 1 and 2 are needed and therefore only those are
returned.
Remove prints and add string post-processing

Added some post-processing to remove unnecessary whitespace and check
that there is only one item per list and then return this item as
string, since there is no point in returning a one-item list.
Also I have decided to return all 4 columns, since this is more stable
to test and easier to work with later on.
Add tests for function make_single_line_snippets

I have added those tests in their own source file in order to keep the
others clean, since I may have to add many more tests,
Add test to test_make_single_line_snippets.py

Add a test to make sure a single line is parsed correctly. Also
whitespace is now trimmed on both sides on all columns