fix: rewrite user-facing messages in Buying module

Conservative cleanup of frappe.throw/msgprint messages per the message style
guide; meaning, severity, and .format() arguments are unchanged:

- index bare {} placeholders as {0}/{1}/... so translators can reorder
- move f-strings / .format() / concatenation out of _() (they break gettext
  extraction and never translate)
- wrap translatable dynamic values (DocType/Select labels) in _()
- fix grammar and colloquialisms
- drop no-op _() wrapping runtime-built strings

Part of #53976.

erpnext/buying/doctype/purchase_order/services/subcontracting.py

@@ -36,7 +36,7 @@ class SubcontractingService:
 							)
 						)
 				if not item.fg_item_qty:
-					frappe.throw(_("Row #{0}: Finished Good Item Qty can not be zero").format(item.idx))
+					frappe.throw(_("Row #{0}: Finished Good Item Qty cannot be zero").format(item.idx))
 		else:
 			for item in doc.items:
 				item.set("fg_item", None)

erpnext/buying/doctype/request_for_quotation/request_for_quotation.py

@@ -421,7 +421,7 @@ def check_portal_enabled(reference_doctype):
 	if not frappe.db.get_value("Portal Menu Item", {"reference_doctype": reference_doctype}, "enabled"):
 		frappe.throw(
 			_(
-				"The Access to Request for Quotation From Portal is Disabled. To Allow Access, Enable it in Portal Settings."
+				"Access to Request for Quotation from the portal is disabled. To allow access, enable it in Portal Settings."
 			)
 		)
 

erpnext/buying/doctype/supplier_scorecard_variable/supplier_scorecard_variable.py

@@ -43,11 +43,11 @@ class SupplierScorecardVariable(Document):
 
 				import_string_path(self.path)
 			except AttributeError:
-				frappe.throw(_("Could not find path for " + self.path), VariablePathNotFound)
+				frappe.throw(_("Could not find path for {0}").format(self.path), VariablePathNotFound)
 
 		else:
 			if not hasattr(sys.modules[__name__], self.path):
-				frappe.throw(_("Could not find path for " + self.path), VariablePathNotFound)
+				frappe.throw(_("Could not find path for {0}").format(self.path), VariablePathNotFound)
 
 
 def get_total_workdays(scorecard):