[ENH] Add a better error message for HTTPClient connection errors (chroma-core#1349)

## Description of changes

*Summarize the changes made by this PR.*
 - Improvements & Bug fixes
- Add a better error message for HTTPClient connection errors during
tenant/db validation on startup
 - New functionality
	 - N/A

Author: HammadB

chromadb/api/client.py

@@ -2,6 +2,7 @@
 from uuid import UUID
 
 from overrides import override
+import requests
 from chromadb.api import AdminAPI, ClientAPI, ServerAPI
 from chromadb.api.types import (
     CollectionMetadata,
@@ -402,13 +403,21 @@ def set_database(self, database: str) -> None:
     def _validate_tenant_database(self, tenant: str, database: str) -> None:
         try:
             self._admin_client.get_tenant(name=tenant)
+        except requests.exceptions.ConnectionError:
+            raise ValueError(
+                "Could not connect to a Chroma server. Are you sure it is running?"
+            )
         except Exception:
             raise ValueError(
                 f"Could not connect to tenant {tenant}. Are you sure it exists?"
             )
 
         try:
             self._admin_client.get_database(name=database, tenant=tenant)
+        except requests.exceptions.ConnectionError:
+            raise ValueError(
+                "Could not connect to a Chroma server. Are you sure it is running?"
+            )
         except Exception:
             raise ValueError(
                 f"Could not connect to database {database} for tenant {tenant}. Are you sure it exists?"