When resolving a tool dep, report back the pants.ini section with a reference that is failing.
Added a convenience method to pull a dependency list from pants.ini and   report the section/key info along to register_jvm_tool()

Testing Done:
ci is baking.

Manual Testing:

1) renamed target 3rdparty:protobuf-2.4.1 to 3rdparty:protobuf-2.4.2 and ran example protobuf compile.

``
$ ./pants goal compile examples/src/java/com/pants/examples/protobuf/distance
  ...
Exception message: protobuf-2.4.1 was not found in BUILD file /Users/zundel/Src/Pants/3rdparty/BUILD. Perhaps you meant one of:
  :checkstyle
  ...
  :util-core
  referenced from [protobuf-gen] key: javadeps in pants.ini
```

2) In BUILD.tools, I renamed '//:jar-tool to //:jar-tool2

```
$ ./pants goal bundle examples/src/java/com/pants/examples/hello/main
  ...
                     Failed to resolve target for bootstrap tool: //:jar-tool. You probably need to add this dep to your tools BUILD file(s), usually located in the root of the build.
                     See pants.ini section: [jar-tool] key: bootstrap-tools
                     Error: jar-tool was not found in BUILD file /Users/zundel/Src/Pants/BUILD. Perhaps you meant one of:
                       BUILD:readme
                       ...
                       BUILD.tools:zinc

```

Bugs closed: 676

Reviewed at https://rbcommons.com/s/twitter/r/1162/
Sync of https://github.com/pantsbuild/pants/commit/7df721222b91950481c2ca120cbef55d8798a245