Downcase first letter of all error messages

Most of them were already lowercase. I just downcase the ones that were
in uppercase.
CheckStructure: cache result of get_structure
Uniformize commit citations in error messages

Error messages that want to refer to a particular commit now begin with
"commit SHA1 ...". If the SHA1 is unknown it's left out.
Uniformize error message endings

They don't end in a period and neither in a newline.