#23 #26 Improve error handling for relation macros (#27)

Restructure relation macros to better handle errors, improving consistency of getting the relation types.

Description of the issue can be found in #23 and #26

Test plan:
The test cases are available in GH Issue #23 and Issue #26

dbt/include/impala/macros/adapters.sql

@@ -196,48 +196,49 @@
 
 {% macro is_relation_present(relation) -%}
   {% set result_set = run_query('show tables in ' ~ relation.schema ~ ' like "' ~ relation.identifier ~ '"') %}
-  {%- if result_set.rows -%}
-    {% do return(true) %}
-  {% else %}
-    {% do return(false) %}
-  {% endif %}
+
+  {% if execute %}
+    {%- for rs in result_set -%}
+      {% do return(true) %}
+    {%- endfor -%}
+  {%- endif -%}
+
+  {% do return(false) %}
 {% endmacro %}
 
 {% macro get_relation_type(relation) -%}
   {% set rel_type = 'table' %}
   {% set relation_exists = is_relation_present(relation) %}
 
-  {%- if not relation_exists -%}
-    {% do return(rel_type) %}
-  {%- endif -%}
-
-  {% set result_set = run_query('describe extended ' ~ relation) %}
+  {%- if relation_exists -%}
+    {% set result_set = run_query('describe extended ' ~ relation) %}
 
-  {% if execute %}
-    {%- for rs in result_set -%}
-      {%- if rs[0].startswith('Table Type') -%}
-        {%- if rs[1].startswith('VIRTUAL_VIEW') -%}
-          {% set rel_type = 'view' %}
-          {% do return(rel_type) %}
-        {%- elif rs[1].startswith('MANAGED_TABLE') -%}
-          {% set rel_type = 'table' %}
-          {% do return(rel_type) %}
-        {%- elif rs[1].startswith('EXTERNAL_TABLE') -%}
-          {% set rel_type = 'table' %}
-          {% do return(rel_type) %}
+    {% if execute %}
+      {%- for rs in result_set -%}
+        {%- if rs[0].startswith('Table Type') -%}
+          {%- if rs[1].startswith('VIRTUAL_VIEW') -%}
+            {% set rel_type = 'view' %}
+            {% do return(rel_type) %}
+          {%- elif rs[1].startswith('MANAGED_TABLE') -%}
+            {% set rel_type = 'table' %}
+            {% do return(rel_type) %}
+          {%- elif rs[1].startswith('EXTERNAL_TABLE') -%}
+            {% set rel_type = 'table' %}
+            {% do return(rel_type) %}
+          {%- endif -%}
         {%- endif -%}
-      {%- endif -%}
-    {%- endfor -%}
-  {% endif %}
-
+      {%- endfor -%}
+    {%- endif -%}
+  {%- endif -%}
+  
   {% do return(rel_type) %}
 {% endmacro %}
 
 {% macro impala__rename_relation(from_relation, to_relation) -%}
   {% set from_rel_type = get_relation_type(from_relation) %}
-
+  
   {% call statement('drop_relation') %}
-    drop {{ to_relation.type }} if exists {{ to_relation }}
+    drop {{ from_rel_type }} if exists {{ to_relation }}
   {% endcall %}
   {% call statement('rename_relation') -%}
     {% if not from_rel_type %}