cmake: Clarify errors when a script cannot be read

Report the path and reason when a file cannot be opened. Particularly,
when under `cmake -P` there is no backtrace to identify the file,
leaving very opaque error messages.

Also avoid reporting a second generic error when we've already reported
that the file does not exist. For cases where the generic error is hit,
reword it for clarity.

Source/cmListFileCache.cxx

@@ -132,7 +132,9 @@ bool cmListFileParser::ParseFile()
   cmListFileLexer_BOM bom;
   if (!cmListFileLexer_SetFileName(this->Lexer.get(), filename->c_str(),
                                    &bom)) {
-    this->IssueFileOpenError("cmListFileCache: error can not open file.");
+    this->IssueFileOpenError(
+      cmStrCat("Cannot open file \"", this->FileName,
+               "\": ", cmSystemTools::GetLastSystemError()));
     return false;
   }
 

Source/cmake.cxx

@@ -898,9 +898,10 @@ void cmake::ReadListFile(std::vector<std::string> const& args,
     }
     if (!cmSystemTools::FileExists(path, true)) {
       cmSystemTools::Error("Not a file: " + path);
+      return;
     }
     if (!mf.ReadListFile(path)) {
-      cmSystemTools::Error("Error processing file: " + path);
+      cmSystemTools::Error("Failed to read file: " + path);
     }
   }
 }

Tests/RunCMake/CommandLine/P_no-file-stderr.txt

@@ -1,4 +1,2 @@
 ^CMake Error: Not a file: [^
-]*/Tests/RunCMake/CommandLine/P_no-file-build/nosuchscriptfile\.cmake
-CMake Error: Error processing file: [^
 ]*/Tests/RunCMake/CommandLine/P_no-file-build/nosuchscriptfile\.cmake$

Tests/RunCMake/CommandLine/P_parse-error-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CommandLine/P_parse-error-stderr.txt

@@ -0,0 +1,6 @@
+^CMake Error at [^
+]*/P_parse-error\.cmake:2:
+  Parse error\.  Function missing ending "\)"\.  End of file reached\.
++
+CMake Error: Failed to read file: [^
+]*/Tests/RunCMake/CommandLine/P_parse-error\.cmake$

Tests/RunCMake/CommandLine/P_parse-error.cmake

@@ -0,0 +1,2 @@
+message(STATUS "before")
+foo(

Tests/RunCMake/CommandLine/RunCMakeTest.cmake

@@ -123,6 +123,7 @@ run_cmake_command(G_no-arg ${CMAKE_COMMAND} -B DummyBuildDir -G)
 run_cmake_command(G_bad-arg ${CMAKE_COMMAND} -B DummyBuildDir -G NoSuchGenerator)
 run_cmake_command(P_no-arg ${CMAKE_COMMAND} -P)
 run_cmake_command(P_no-file ${CMAKE_COMMAND} -P nosuchscriptfile.cmake)
+run_cmake_command(P_parse-error ${CMAKE_COMMAND} -P "${RunCMake_SOURCE_DIR}/P_parse-error.cmake")
 run_cmake_command(P_args ${CMAKE_COMMAND} -P "${RunCMake_SOURCE_DIR}/P_args.cmake" relative/path "${RunCMake_SOURCE_DIR}")
 run_cmake_command(P_arbitrary_args ${CMAKE_COMMAND} -P "${RunCMake_SOURCE_DIR}/P_arbitrary_args.cmake" -- -DFOO -S -B --fresh --version)
 run_cmake_command(P_P_in_arbitrary_args ${CMAKE_COMMAND} -P "${RunCMake_SOURCE_DIR}/P_arbitrary_args.cmake" -- -P "${RunCMake_SOURCE_DIR}/non_existing.cmake")