Check exit code of LaTeX process in LaTeX::run

Systemcall::startscript returns the exit code of the LaTeX command
that is run, but the return value was not being checked by
LaTeX::run.  Instead, we relied on parsing log files. However, this
parsing is not perfect.

The return value is now checked and if the exit code of the command
is non-zero, an enum value is added to the return and the user is
notified of the error.

At a higher level, if the LaTeX command returns a non-zero exit code,
in the GUI a message such as
"Error while exporting format: PDF (LuaTeX)" will be given instead of
"Successful preview of format: PDF (LuaTeX)".

When run on the commandline, lyx -e lualatex example.lyx
will give "Error: LaTeX failed" and a non-zero exit code
where before it gave a zero exit code.

A real example of the bug this commit fixes is LyX's (as of this commit)
ACM-sigplan.lyx template.
Before this commit:
  $ lyx -e pdf2 ACM-sigplan.lyx
  [...snip...]
  support/Systemcall.cpp (288): Systemcall: 'pdflatex  "ACM-sigplan.tex"'
  finished with exit code 1
  $ echo $?
  0
Starting with this commit:
  $ mylyx master -e pdf2 ACM-sigplan.lyx
  support/Systemcall.cpp (288): Systemcall: 'pdflatex  "ACM-sigplan.tex"'
  finished with exit code 1
  Error: LaTeX failed
  ----------------------------------------
  LaTeX did not run successfully. The command that was run exited with
  error.
  $ echo $?
  1

RELEASE-NOTES

@@ -52,6 +52,8 @@ Changes with respect to external programs and libraries in 2.2:
   run LyX against Qt 4.8.x. On Windows, Qt 4.8.5 suffers from a bug that 
   breaks the use of shortcuts, so on Windows Qt 4.8.4 is advised.
 
+- LyX now gives an error if the underlying LaTeX command exited with error.
+
 
 Known issues in version 2.2.0
 -----------------------------

src/Converter.cpp

@@ -660,6 +660,14 @@ bool Converters::runLaTeX(Buffer const & buffer, string const & command,
 					       "Additionally, LyX could not locate "
 					       "the LaTeX log %1$s."), from_utf8(name));
 		Alert::error(_("LaTeX failed"), str);
+	} else if (result & LaTeX::NONZERO_ERROR) {
+		docstring const str =
+			bformat(_( "The external program\n%1$s\n"
+			      "finished with an error. "
+			      "It is recommended you fix the cause of the external "
+			      "program's error (check the logs). "), from_utf8(command));
+		// FIXME: In LyX 2.3.0 the warning will be converted to an error.
+		Alert::error(_("LaTeX failed"), str);
 	} else if (result & LaTeX::NO_OUTPUT) {
 		Alert::warning(_("Output is empty"),
 			       _("No output file was generated."));

src/LaTeX.cpp

@@ -226,7 +226,8 @@ int LaTeX::run(TeXErrors & terr)
 	LYXERR(Debug::LATEX, "Run #" << count);
 	message(runMessage(count));
 
-	startscript();
+	int const exit_code = startscript();
+
 	scanres = scanLogFile(terr);
 	if (scanres & ERROR_RERUN) {
 		LYXERR(Debug::LATEX, "Rerunning LaTeX");
@@ -422,6 +423,7 @@ int LaTeX::run(TeXErrors & terr)
 
 	// Write the dependencies to file.
 	head.write(depfile);
+
 	if (scanres & NO_OUTPUT) {
 		// A previous run could have left a PDF and since
 		// no PDF is created if NO_OUTPUT, we remove any
@@ -431,6 +433,10 @@ int LaTeX::run(TeXErrors & terr)
 		// be the same so any lingering PDF will be viewed.
 		deleteFilesOnError();
 	}
+
+	if (exit_code)
+		scanres |= NONZERO_ERROR;
+
 	LYXERR(Debug::LATEX, "Done.");
 	return scanres;
 }

src/LaTeX.h

@@ -138,10 +138,12 @@ public:
 		///
 		BIBTEX_ERROR = 16384,
 		///
+		NONZERO_ERROR = 32768, // the command exited with nonzero status
+		///
 		//FIXME: BIBTEX_ERROR has been removed from ERRORS for now, since users were irritated
 		//       about those errors which prevented compilation of previously compiling documents.
 		//       Think about a "gentle" transfer to BibTeX error reporting.
-		ERRORS = TEX_ERROR + LATEX_ERROR,
+		ERRORS = TEX_ERROR + LATEX_ERROR + NONZERO_ERROR,
 		///
 		WARNINGS = TEX_WARNING + LATEX_WARNING + PACKAGE_WARNING
 	};