Show a more helpful error message when plugins cannot be imported
### Describe the bug
If a dbt plugin cannot be imported, dbt will show a message about invalid credentials, eg:
```
Running with dbt=0.14.3
Error importing adapter: cannot import name 'collections_abc'
Encountered an error while reading profiles:
  ERROR Runtime Error
  Credentials in profile "<profile name>", target "dev" invalid: Runtime Error
    Could not find adapter type bigquery!
Defined profiles:
 - ....
```

If this is an error that occurs at import-time, dbt should not indicate that the credentials are invalid. dbt should also not show a list of defined profiles in this error message.

Desired output:
```
Running with dbt=0.14.3
The dbt plugin 'bigquery' could not be imported: cannot import name 'collections_abc'
```

dbt version: v0.14.3. I need to test against 0.15.0 to determine if this behavior has recently changed.
diff --git a/core/dbt/adapters/factory.py b/core/dbt/adapters/factory.py
index 0c8edd272..0742a1e0c 100644
--- a/core/dbt/adapters/factory.py
+++ b/core/dbt/adapters/factory.py
@@ -46,16 +46,17 @@ class AdpaterContainer:
         # singletons
         try:
             mod = import_module('.' + name, 'dbt.adapters')
-        except ImportError as e:
-            logger.info("Error importing adapter: {}".format(e))
-            raise RuntimeException(
-                "Could not find adapter type {}!".format(name)
-            )
-        if not hasattr(mod, 'Plugin'):
-            raise RuntimeException(
-                f'Could not find plugin in {name} plugin module'
-            )
-        plugin: AdapterPlugin = mod.Plugin  # type: ignore
+        except ModuleNotFoundError as exc:
+            # if we failed to import the target module in particular, inform
+            # the user about it via a runtiem error
+            logger.info(f'Error importing adapter: {exc}')
+            if exc.name == 'dbt.adapters.' + name:
+                raise RuntimeException(f'Could not find adapter type {name}!')
+            # otherwise, the error had to have come from some underlying
+            # library. Log the stack trace.
+            logger.debug('', exc_info=True)
+            raise
+        plugin = mod.Plugin  # type: AdapterPlugin
         plugin_type = plugin.adapter.type()
 
         if plugin_type != name:
['core/dbt/adapters/factory.py']