[sancov] better input parameters validation

test/tools/sancov/AArch64/print_coverage_pcs.test

 REQUIRES: aarch64-registered-target
 RUN: not sancov -print-coverage-pcs %p/../Inputs/test-linux_android_aarch64 2>&1 | FileCheck %s --check-prefix=AARCH64
 
-AARCH64: Error: __sanitizer_cov* functions not found
+AARCH64: ERROR: __sanitizer_cov* functions not found

test/tools/sancov/validation.test

+REQUIRES: x86_64-linux
+RUN: not sancov -covered-functions %p/Inputs/test-linux_x86_64 2>&1 | FileCheck --check-prefix=NOCFILE %s
+
+NOCFILE: WARNING: No coverage file for {{.*}}test-linux_x86_64
+NOCFILE: ERROR: No valid coverage files given.
+

tools/sancov/sancov.cc

struct CoverageStats {
   size_t CovPoints;
   size_t AllFns;
   size_t CovFns;
 };
 
 // --------- ERROR HANDLING ---------
 
 static void fail(const llvm::Twine &E) {
-  errs() << "Error: " << E << "\n";
+  errs() << "ERROR: " << E << "\n";
   exit(1);
 }
 
 static void failIf(bool B, const llvm::Twine &E) {
   if (B)
     fail(E);
 }
 
 static void failIfError(std::error_code Error) {
   if (!Error)
     return;
-  errs() << "Error: " << Error.message() << "(" << Error.value() << ")\n";
+  errs() << "ERROR: " << Error.message() << "(" << Error.value() << ")\n";
   exit(1);
 }
 
 template <typename T> static void failIfError(const ErrorOr<T> &E) {
   failIfError(E.getError());
 }
 
 static void failIfError(Error Err) {
   if (Err) {
-    logAllUnhandledErrors(std::move(Err), errs(), "Error: ");
+    logAllUnhandledErrors(std::move(Err), errs(), "ERROR: ");
     exit(1);
   }
 }
 
 template <typename T> static void failIfError(Expected<T> &E) {
   failIfError(E.takeError());
 }
 
for (const auto &FileName : FileNames) {
     if (!Cov)
       continue;
     OS << *Cov.get();
   }
 }
 
 static std::unique_ptr<SymbolizedCoverage>
 merge(const std::vector<std::unique_ptr<SymbolizedCoverage>> &Coverages) {
+  if (Coverages.empty())
+    return nullptr;
+
   auto Result = make_unique<SymbolizedCoverage>();
 
   for (size_t I = 0; I < Coverages.size(); ++I) {
     const SymbolizedCoverage &Coverage = *Coverages[I];
     std::string Prefix;
     if (Coverages.size() > 1) {
       // prefix is not needed when there's only one file.
       Prefix = utostr(I);
for (const auto &FileName : CovFiles) {
       auto Iter = ObjFiles.find(Components[1]);
       if (Iter == ObjFiles.end()) {
         fail("Object file for coverage not found: " + FileName);
       }
 
       CoverageByObjFile[Iter->second].push_back(FileName);
     };
 
+    for (const auto &Pair : ObjFiles) {
+      auto FileName = Pair.second;
+      if (CoverageByObjFile.find(FileName) == CoverageByObjFile.end())
+        errs() << "WARNING: No coverage file for " << FileName << "\n";
+    }
+
     // Read raw coverage and symbolize it.
     for (const auto &Pair : CoverageByObjFile) {
       if (findSanitizerCovFunctions(Pair.first).empty()) {
         errs()
-            << "Ignoring " << Pair.first
+            << "WARNING: Ignoring " << Pair.first
             << " and its coverage because  __sanitizer_cov* functions were not "
                "found.\n";
         continue;
       }
 
       for (const std::string &CoverageFile : Pair.second) {
         auto DataOrError = RawCoverage::read(CoverageFile);
         failIfError(DataOrError);