Informative error messages, update README

ProtoLogTool error messages are hard to read, this CL addresses this
issue adding more info (including file name and line number) and using
verbose exception messages where available.

Updated README with recent ProtoLog changes.

Bug: 141672033
Test: atest protologtool-tests
Change-Id: Ibfca908808526adb6c8c5f68893f093a8233728f