Handle a number of bad failure-path behaviors (#1807)

[beckjake]

Fixes #1807

This is one of those bugs where I don't see the bad behavior anymore so I assume it's ok. This was hard to write tests for, I had to use our internal analytics project with 8 threads to reproduce the issue: I assume it needs a large and complex test case to trigger. And I don't know where I'd even pull debugging information from.

Anyway, my attempts at fixing the issue described:

• When a connection is missing, raise a special exception
  • instead of RuntimeError, this derives from our RuntimeException, so we catch/suppress it better
• Be graceful if a connection does not exist during nice_connection_name
• Handle the case where exceptions caught by the Snowflake exception handler do not have a 'msg' attr
• Re-raise exceptions in the adapter exception handlers "from" the originating error so we don't get big double stack traces about "during handling... another exception was raised"

[clrcrl]

Tested this out with the problematic dbt project, and it LGTM:

on dev/louisa-may-alcott

$ dbt compile
Running with dbt=0.15.0-a1
Found 106 models, 71 tests, 1 snapshot, 0 analyses, 278 macros, 1 operation, 6 seed files, 23 sources

10:00:14 | Concurrency: 8 threads (target='dev')
10:00:14 |
Unhandled error while executing model.fishtown_internal_analytics.stg_dbt_platform_contexts
connection never acquired for thread (38571, 123145532657664), have []
Unhandled error while executing model.fishtown_internal_analytics.stg_snowplow_web_page_contexts
connection never acquired for thread (38571, 123145527402496), have []
Unhandled error while executing model.fishtown_internal_analytics.stg_dbt_invocation_env_contexts
connection never acquired for thread (38571, 123145495871488), have []
Unhandled error while executing model.fishtown_internal_analytics.stg_dbt_run_model_contexts
connection never acquired for thread (38571, 123145511636992), have []
Encountered an error:
Runtime Error
  Database Error in model fct_opportunities (models/marts/sales/fct_opportunities.sql)
    002003 (02000): SQL compilation error:
    Schema 'ANALYTICS.DBT_CLAIRE' does not exist.

on fix/snowflake-query-bad-errors

$ dbt compile
Running with dbt=0.15.0-a1
Found 106 models, 71 tests, 1 snapshot, 0 analyses, 278 macros, 1 operation, 6 seed files, 23 sources

09:59:04 | Concurrency: 8 threads (target='dev')
09:59:04 |
Encountered an error:
Runtime Error
  Database Error in model fct_opportunities (models/marts/sales/fct_opportunities.sql)
    002003 (02000): SQL compilation error:
    Schema 'ANALYTICS.DBT_CLAIRE' does not exist.

[drewbanin]

this LGTM - ship it!