Improve error messages and handling, fix bugs in packs actions

Previously if a pack didn't exist or didn't have a pack.yaml file we returned a very cryptic error - `Exception: No packs from [u'packname'] were downloaded.` error message.

This pull request fixes and improves the message, but we still need to improve the error formatting and reporting. Currently it's still *very* unfriendly because we throw an exception and not return a result. In fact, this should be our top priority - currently, the experience is very bad if a packs action fails.

```
+-----------------+--------------------------------------------------------------+
| Property        | Value                                                        |
+-----------------+--------------------------------------------------------------+
| id              | 5492e47d0640fd0b7cdaf8d1                                     |
| context         | {                                                            |
|                 |     "user": "stanley"                                        |
|                 | }                                                            |
| parameters      | {                                                            |
|                 |     "packs": [                                               |
|                 |         "fooo",                                              |
|                 |         "barr"                                               |
|                 |     ]                                                        |
|                 | }                                                            |
| status          | failed                                                       |
| start_timestamp | 2014-12-18T14:28:13.214000Z                                  |
| result          | {                                                            |
|                 |     "result": null,                                          |
|                 |     "exit_code": 1,                                          |
|                 |     "stderr": "Traceback (most recent call last):            |
|                 |   File "/data/stanley/st2actions/st2actions/runners/python_a |
|                 | ction_wrapper.py", line 106, in <module>                     |
|                 |     obj.run()                                                |
|                 |   File "/data/stanley/st2actions/st2actions/runners/python_a |
|                 | ction_wrapper.py", line 57, in run                           |
|                 |     output = action.run(**self._parameters)                  |
|                 |   File                                                       |
|                 | "/data/stanley/contrib/packs/actions/pack_mgmt/download.py", |
|                 | line 24, in run                                              |
|                 |     return self._validate_result(result=result, packs=packs, |
|                 | repo_url=repo_url)                                           |
|                 |   File                                                       |
|                 | "/data/stanley/contrib/packs/actions/pack_mgmt/download.py", |
|                 | line 96, in _validate_result                                 |
|                 |     raise Exception(message)                                 |
|                 | Exception: No packs were downloaded from repository          |
|                 | "https://github.com/StackStorm/st2contrib.git".              |
|                 |                                                              |
|                 | Errors:                                                      |
|                 |  - fooo: Failure : Pack "fooo" not found or it's missing a   |
|                 | "pack.yaml" file.                                            |
|                 |  - barr: Failure : Pack "barr" not found or it's missing a   |
|                 | "pack.yaml" file.                                            |
|                 | ",                                                           |
|                 |     "stdout": ""                                             |
|                 | }                                                            |
| action          | packs.download                                               |
| callback        |                                                              |
| end_timestamp   | 2014-12-18T14:28:30.651000Z                                  |
+-----------------+--------------------------------------------------------------+
```

On top of that, it also fixes some other issues.