standardize commit logging and return on edge and node writers

[dosumis]

• As usual, every method should return something useful or False
• KB_tools write objects accumulate a stack of cypher statements until .commit() is called.
• KB_tools Writer object commits should return False if any part of the commit stack has failed
• Error messages that indicate failure of a statement in the commit stack are logged (not returned by the commit statement).
• If the error log has any content, then commit returns False, otherwise it returns True. It is the responsibility of the calling script to check for False return and then check the log.
• We need a mechanism to reliably clear the error log between commits. As we are only logging commit-time errors, this can happen at the beginning of a commit. This has the disadvantage that the old error log hangs around until the next commit.
• .get_log => returns log and clears it
• pattern error log is simply a concatenation of edge and node writer error logs.

[dosumis]

Co-ordinate with work on curation repo!