back.verilog: better error message if Yosys is not found.

Fixes #17.

nmigen/back/verilog.py

@@ -12,12 +12,21 @@ class YosysError(Exception):
 
 
 def convert(*args, **kwargs):
-    il_text = rtlil.convert(*args, **kwargs)
+    try:
         popen = subprocess.Popen([os.getenv("YOSYS", "yosys"), "-q", "-"],
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
             encoding="utf-8")
+    except FileNotFoundError as e:
+        if os.getenv("YOSYS"):
+            raise YosysError("Could not find Yosys in {} as specified via the YOSYS environment "
+                             "variable".format(os.getenv("YOSYS"))) from e
+        else:
+            raise YosysError("Could not find Yosys in PATH. Place `yosys` in PATH or specify "
+                             "path explicitly via the YOSYS environment variable") from e
+
+    il_text = rtlil.convert(*args, **kwargs)
     verilog_text, error = popen.communicate("""
 # Convert nMigen's RTLIL to readable Verilog.
 read_ilang <<rtlil