Unhelpful error message with malformed packages.yml
### Describe the bug
```
MacBook-Pro:analytics tristan$ cat packages.yml
packages:
  - package: fishtown-analytics/stripe
    revision: 0.3.1
```

I had recently switched from a git reference to this package to a hub reference, and forgot that I needed to change from `revision` to `version`. This is the error message I received, which made it significantly more challenging to debug:

```
MacBook-Pro:analytics tristan$ dbt clean && dbt deps
Running with dbt=0.15.0
Encountered an error while reading the project:
  ERROR: Runtime Error
  Invalid package config: at path []: 'local' is a required property
```

### Expected behavior
I would expect an error message like:
```
Required parameter version not supplied.
```
...or something similar.

**The output of `dbt --version`:**
```
MacBook-Pro:analytics tristan$ dbt --version
installed version: 0.15.0
   latest version: 0.15.0

Up to date!
```
diff --git a/core/dbt/config/project.py b/core/dbt/config/project.py
index 620aa0773..3ba025fa7 100644
--- a/core/dbt/config/project.py
+++ b/core/dbt/config/project.py
@@ -59,6 +59,17 @@ Check the requirements for the '{package}' package, or run dbt again with \\
 --no-version-check
 """
 
+MALFORMED_PACKAGE_ERROR = """\\
+The packages.yml file in this project is malformed. Please double check
+the contents of this file and fix any errors before retrying.
+
+You can find more information on the syntax for this file here:
+https://docs.getdbt.com/docs/package-management
+
+Validator Error:
+{error}
+"""
+
 
 def _list_if_none(value):
     if value is None:
@@ -130,7 +141,7 @@ def package_config_from_data(packages_data):
         packages = PackageConfig.from_dict(packages_data)
     except ValidationError as e:
         raise DbtProjectError(
-            'Invalid package config: {}'.format(validator_error_message(e))
+            MALFORMED_PACKAGE_ERROR.format(error=str(e.message))
         ) from e
     return packages
 
['core/dbt/config/project.py']