Process input files in Standard JSON mode just like in other modes

- This makes `-` for stdin work.
- `--ignore-missing` now works with `--standard-json`, though it's not very useful because there can be at most one input file.
- Separate errors for situations where there are no input files on the command line (this can be detected in the parser) and where they are not present on disk.

solc/main.cpp

@@ -57,6 +57,8 @@ int main(int argc, char** argv)
 	solidity::frontend::CommandLineInterface cli(cin, cout, cerr);
 	if (!cli.parseArguments(argc, argv))
 		return 1;
+	if (!cli.readInputFiles())
+		return 1;
 	if (!cli.processInput())
 		return 1;
 	bool success = false;