obsolete: refactor writemarkers to only encode them
The function is now able to write the version header as necessary. The function
now yield bytes to be written to a stream.
This should ease later use of this function for wireprotocol based exchanged.
Prepare the public use of the writemarker by wireprotocol function.
hooks: print out more information when loading a python hook fails
When loading a python hook with file syntax fails, there is no
information that this happened while loading a hook. When the python
file does not exist even the file name is not printed. (Only that a
file is missing.)
This patch adds this information and a test for loading a non existing file and
a directory not being a python module.