config: Better error messages when opening files

If a config file is a directory, ifstream would just read it as an empty
file without any errors.

Failing early here is a good idea.

src/components/config_parser.cpp

@@ -1,8 +1,13 @@
 #include "components/config_parser.hpp"
 
 #include <algorithm>
+#include <cerrno>
+#include <cstring>
 #include <fstream>
 
+#include "utils/file.hpp"
+#include "utils/string.hpp"
+
 POLYBAR_NS
 
 config_parser::config_parser(const logger& logger, string&& file, string&& bar)
@@ -89,6 +94,14 @@ void config_parser::parse_file(const string& file, file_list path) {
     throw application_error("include-file: Dependency cycle detected:\n" + path_str);
   }
 
+  if (!file_util::exists(file)) {
+    throw application_error("Failed to open config file " + file + ": " + strerror(errno));
+  }
+
+  if (!file_util::is_file(file)) {
+    throw application_error("Config file " + file + " is not a file");
+  }
+
   m_log.trace("config_parser: Parsing %s", file);
 
   int file_index;