Produce explanatory error messages on lexer/parser failure

[tilowiklundSensmetry]

Currently lexing/parsing errors produce very short errors, without reference to the source code. This means that sysand include and sysand build may fail in ways that are hard to understand and debug for the user.

Error messages should at a minimum show the offending part of the code, in which file it appears, and ideally contain a longer, human readable, explanation of the error.

[tilowiklundSensmetry]

Various improvements in #87, but still requires floating everything to the CLI and getting rid of a few implicit panics.