This modifies the panic feature to accept a boolean or a string. If true
or not empty, then it will cause the panic. This makes some of the error
code a little less ugly.
This is a newer implementation of the panic magic. I kept the old commit
in for posterity and to show the difference. The two versions are
identical to the end-user with one exception: the newer version doesn't
include a useless panic resource in the graph when there is no panic. In
this version, the panic function returns false and the if statement it's
the condition of, doesn't produce the resource within. On error, we
still consume the function in the if expression, and doing so causes
everything to shutdown.
The other benefit is that the implementation is much cleaner and doesn't
need the interpolate hack.
It's valuable to check your runtime values and to shut down the entire
engine in case something doesn't match. This patch adds some magic
plumbing to support a "panic" mechanism.
A new "panic" statement gets transparently converted into a panic
function and panic resource. The former errors if the input is not
empty. The latter must be present to consume the value, but doesn't
actually do anything.
