Inconsistent use of exceptions

[kyllingstad]

The way we're currently using exceptions (or not using exceptions) is very inconsistent now. Here are some common design decisions where I'd like us to take a more structured approach:

• When to use a std::runtime_error-derived exception versus a std::logic_error-derived one
• When to use our dedicated cse::error exception type versus a std exception type, and which error codes to use in which situations
• When to use CSE_PANIC() versus throwing a std::logic_error-derived exception