Remove dc_open call

A new context is now created by calling Context::new and therefore you
always have a valid context.  This is much more in Rust style and will
allow a lot of furture simplifications on the context itself.

The FFI layer has not yet been adjusted in this commit and thus will
fail.

src/error.rs

@@ -22,6 +22,8 @@ pub enum Error {
     Image(image_meta::ImageError),
     #[fail(display = "{:?}", _0)]
     Utf8(std::str::Utf8Error),
+    #[fail(display = "{:?}", _0)]
+    CStringError(crate::dc_tools::CStringError),
 }
 
 pub type Result<T> = std::result::Result<T, Error>;
@@ -62,6 +64,12 @@ impl From<image_meta::ImageError> for Error {
     }
 }
 
+impl From<crate::dc_tools::CStringError> for Error {
+    fn from(err: crate::dc_tools::CStringError) -> Error {
+        Error::CStringError(err)
+    }
+}
+
 #[macro_export]
 macro_rules! bail {
     ($e:expr) => {