[FIR] Improve NO_ELSE_IN_WHEN message

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticRenderers.kt

@@ -169,11 +169,11 @@ object FirDiagnosticRenderers {
 
     val WHEN_MISSING_CASES = Renderer { missingCases: List<WhenMissingCase> ->
         if (missingCases.singleOrNull() == WhenMissingCase.Unknown) {
-            "'else' branch"
+            "an 'else' branch"
         } else {
             val list = missingCases.joinToString(", ", limit = WHEN_MISSING_LIMIT) { "'$it'" }
             val branches = if (missingCases.size > 1) "branches" else "branch"
-            "$list $branches or 'else' branch instead"
+            "the $list $branches or an 'else' branch"
         }
     }
 

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirErrorsDefaultMessages.kt

@@ -2228,7 +2228,7 @@ object FirErrorsDefaultMessages : BaseDiagnosticRendererFactory() {
 
         // When expressions
         map.put(EXPECTED_CONDITION, "Condition of type 'Boolean' expected.")
-        map.put(NO_ELSE_IN_WHEN, "''when'' expression must be exhaustive. Add the necessary {0}{1}", WHEN_MISSING_CASES, STRING)
+        map.put(NO_ELSE_IN_WHEN, "''when'' expression must be exhaustive. Add {0}{1}.", WHEN_MISSING_CASES, STRING)
         map.put(INVALID_IF_AS_EXPRESSION, "'if' must have both main and 'else' branches when used as an expression.")
         map.put(
             NON_EXHAUSTIVE_WHEN_STATEMENT,

compiler/testData/cli/jvm/returnAsWhenKey.out

@@ -1,4 +1,4 @@
-compiler/testData/cli/jvm/returnAsWhenKey.kt:4:5: error: 'when' expression must be exhaustive. Add the necessary 'true', 'false' branches or 'else' branch instead
+compiler/testData/cli/jvm/returnAsWhenKey.kt:4:5: error: 'when' expression must be exhaustive. Add the 'true', 'false' branches or an 'else' branch.
     when (true) {
     ^
 COMPILATION_ERROR