Design runtime error interface

[rmhsilva]

There are a few aspects to this, but essentially, it need to answer the question: when something doesn't work at runtime in a Teal program, how does one debug the problem?

Error sources:

• Bug in Teal interpreter/runtime
• Logic error in user program (e.g. unbound variable)
• Other

It needs to be obvious when one of these happens (and which kind it is), and reporting Teal bugs needs to be simple. As much information as possible should be reported to the programmer.

[rmhsilva]

Done!