Improves osproc.startProcess error message on POSIX. Ref #2183.

lib/pure/osproc.nim

@@ -785,7 +785,8 @@ elif not defined(useNimRtl):
     var error: cint
     let sizeRead = read(data.pErrorPipe[readIdx], addr error, sizeof(error))
     if sizeRead == sizeof(error):
-      raiseOSError($strerror(error))
+      raiseOSError("Could not find command: '$1'. OS error: $2" %
+          [$data.sysCommand, $strerror(error)])
 
     return pid