This is a work in progress PR for addressing GH issue 23 and 26

The relevant macros are restructured to handle error better.

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

dbt/include/impala/macros/adapters.sql

@@ -196,40 +196,41 @@
 
 {% 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 %}