WikiArtifact instances also have provides; limit ivy to jvm

This should never get hit unless you accidentally do goal compile on a
wiki -- but maybe you do compile ...:: and hit it.

Testing Done:
Ran tests.

Reviewed at https://rbcommons.com/s/twitter/r/1259/
Refactor artifact cache exceptions.

Presently, the artifact cache will treat any exception during the
insertion process as a non-fatal error.  But some errors should be
fatal -- any sort of misconfiguration, or an attempt to cache
nonexistent files, or an exception thrown while moving files around.
Also remove ArtifactCache.CacheError, as it was not clear when
to use ArtifactCache.CacheError and when to use ArtifactCacheError.

Reviewed at https://rbcommons.com/s/twitter/r/1261/
included code snippets: don't just slap in a pre. syntax highlighting

The pygments code-syntax-highlighting code has some built-in functionality to guess at a code snippet's language based on filename and content. We hadn't hooked it up. This hooks it up.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39542386

patched this and https://rbcommons.com/s/twitter/r/1191/ into a branch and test-generated the doc site: http://pantsbuild.github.io/sitegen-test/JVMProjects.html

Bugs closed: 693

Reviewed at https://rbcommons.com/s/twitter/r/1182/
Fix error message: add missing space

Reviewed at https://rbcommons.com/s/twitter/r/1266/
Singleton for requests session

Instead of instance-level, use a module level singleton so that in multiproc code,
subprocesses each have their own, reusable session.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39542444 passed

Bugs closed: 751

Reviewed at https://rbcommons.com/s/twitter/r/1264/
sitegen: get page-level table-of-contents working again

workaround for: modifying the AST to add the here-links had the side effect
that the find-every-header function no longer finds headings. So reorder
operations: find every header before adding the here-links

While adding an import to test_sitegen.py , noticed it was missing the
usual "from __future__ import", so added that.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39653188

patched this and https://rbcommons.com/s/twitter/r/1182/ into a branch and test-generated the doc site: http://pantsbuild.github.io/sitegen-test/JVMProjects.html

Reviewed at https://rbcommons.com/s/twitter/r/1191/
Merge branch 'master' into rst_to_md_moar