Replace fail invocations with ice_condition.raise in rustc

[brson]

We should see if conditions can suit our purposes; we may not be able to get rid of every fail! invocation that rustc can reach, but it would not hurt to try.

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Original title: Replace fail with session.bug in rustc

When possible rustc should be using session.bug or session.span_bug instead of fail because that prints out proper error messages.

[brson]

Or, if it's a real error condition of course it should use fatal or error

[marijnh]

I really think we should wire things up in a way that allows us to print out the error message from normal failure. Passing sessions to things that otherwise don't take sessions, just to be able to call session.bug, doesn't sound attractive. Not to mention that we're also calling library routines that can fail.

[brson]

I'm inclined to agree. We need failure and logging to be more powerful.

[catamorphism]

Is it reasonable to consider this bug "postponed until error/logging infrastructure improves"?

[pnkfelix]

1. The documentation in the comment above "pub fn monitor" in rustc.rc was quite helpful for me to get my head around our current setup.
2. brson says we can probably close this, as "its probably not worth the effort to expunge all calls to fail! at this point"; however, marijnh's comment makes me pause before closing. Perhaps this represents some sort of weakness, if there really is not a way to make at least a best-effort attempt to extract the string passed to fail! (if any) from the parent task of the failing subtask...

[graydon]

This is an ideal candidate for conditions.

[pnkfelix]

I've got a sketch started over here: https://github.com/pnkfelix/rust/commits/issue1928-add-ice-condition

(Most of the redundancies that I identified while doing it come from known problems in our macro and condition system.)

[pnkfelix]

Note that some issues with conditions may need to be resolved first; e.g. #5446.

[thestinger]

This is still an open issue.

[catamorphism]

Low, not 1.0. (Can be closed if we remove conditions.)

[alexcrichton]

Conditions have been removed.