Improve CHECK-LABEL error message (#26)

I was debugging a CHECK-LABEL related problem and struggled because I
was trying to fix the wrong label in my file. This improved error
message should help in those situations.

filecheck/preprocess.py

@@ -29,7 +29,9 @@ def preprocess_label(self, op: CheckOp):
         pattern, _ = compile_uops(op, dict(), self.opts)
         match = self.input.find_between(pattern, self.range)
         if not match:
-            raise CheckError(f"{op.check_name}: Could not find label in input", op)
+            raise CheckError(
+                f"{op.check_name}: Could not find label '{op.arg}' in input", op
+            )
 
         self.range = self.range.split_at(match)
         self.input.ranges.append(self.range)

tests/filecheck/diagnostics/label-error.test

@@ -0,0 +1,16 @@
+// RUN: strip-comments.sh %s | exfail filecheck %s --comment-prefixes=RUN,COM,DIAG | filecheck %s --check-prefix=DIAG
+
+hello:
+    a
+    b
+
+world:
+    c
+    d
+
+// CHECK-LABEL: hello
+// CHECK-NOT: d
+// CHECK-LABEL: woupsrld
+// CHECK: d
+
+// DIAG: Could not find label 'woupsrld' in input