Improve error message when SESSION_ID doesn't match an actual session (#3121)

Author: benilovj

config/locales/en.yml

@@ -300,7 +300,7 @@ en:
             school_urn:
               inclusion: The school URN is not recognised. If you’ve checked the URN, and you believe it’s valid, contact our support organisation.
             session_id:
-              inclusion: Enter a valid ID for this organisation and programme
+              inclusion: The session ID is not recognised. Download the offline spreadsheet and copy the session ID for this row from there, or contact our support organisation.
             time_of_vaccination:
               blank: Enter a time in the correct format
               less_than_or_equal_to: Enter a time in the past

spec/models/immunisation_import_row_spec.rb

@@ -307,7 +307,8 @@
       it "has errors" do
         expect(immunisation_import_row).to be_invalid
         expect(immunisation_import_row.errors[:session_id]).to include(
-          "Enter a valid ID for this organisation and programme"
+          "The session ID is not recognised. Download the offline spreadsheet and copy the session ID " \
+            "for this row from there, or contact our support organisation."
         )
       end
     end
@@ -318,7 +319,8 @@
       it "has errors" do
         expect(immunisation_import_row).to be_invalid
         expect(immunisation_import_row.errors[:session_id]).to include(
-          "Enter a valid ID for this organisation and programme"
+          "The session ID is not recognised. Download the offline spreadsheet and copy the session ID " \
+            "for this row from there, or contact our support organisation."
         )
       end
     end