rustc: rename the lint `level` enum for style.

CamelCase all the way!
rustc: ng -> gn, fix a typo in a string.
rustc: improve error messages from wrong --pretty flowgraph use.

This defers to .fatal and .span_fatal for errors (rather than `fail!`
which prints the ICE message). It also adds the span lookup when an id
doesn't correspond to a block, to show what it is pointing at.

It also makes the argument parser slightly looser, so that passing
`--pretty flowgraph` recognises the `flowgraph` part and suggests to use
an integer.
auto merge of #14326 : huonw/rust/tiny-fixes, r=pnkfelix

The changes to flowgraph make invalid invocations slightly more forgiving by (trying to) provide slightly more information and by avoiding the ICE message.