Show a more helpful error message when plugins cannot be imported

[drewbanin]

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.

[drewbanin]

closed by #2022