Output an appropriate warning when a cross info file does not exist.

If making a typo, it used to output:
>  Cross info file must have either host or a target machine.
This was not useful at all and looked like there could be a file format
error or some other issue with the content. Let's have an appropriate
error:
> File not found: /some/path

1 files changed, 6 insertions, 1 deletions

diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index 1e4e04b..07489a1 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -907,7 +907,12 @@ class CrossBuildInfo:
 
     def parse_datafile(self, filename):
         config = configparser.ConfigParser()
-        config.read(filename)
+        try:
+            f = open(filename, 'r')
+            config.read_file(f, filename)
+            f.close()
+        except FileNotFoundError:
+            raise EnvironmentException('File not found: %s.' % filename)
         # This is a bit hackish at the moment.
         for s in config.sections():
             self.config[s] = {}