Improve error messages

[protobird-git]

for malformed .stl files

[mbjorge]

High level idea: in p_error, use parser.symstack to look at the stack, and then do some pattern matching to figure out root cause of the parsing error

[protobird-git]

2017 Q3 key result is being tracked by (OKR 2017 Q3) For 5 types of errors, display the line/file of root cause