errors should provide relevant runtime context

[timotheecour]

eg:
assert(!rhs.absolute, "Trying to append absolute path);
is impossible to act upon.
=>
assert(!rhs.absolute, rhs.toString);
is less to type and much more useful; the remaining explanation is less important.

[WebFreak001]

this has been added to the compilers, can be used using dflags "-checkaction=context"

There is an issue open to make this a build option: #2296