Improve errors for missing shell file on Windows

The ConPTY had a cryptic 'assertion failed: success' error whenever
spawning the shell failed.

This changes the ConPTY assertion to a human-readable error which
clearly states that the file could not be found.

The WinPTY backend has not been fixed yet.

This fixes #2016.

src/tty/windows/conpty.rs

@@ -15,6 +15,7 @@
 use super::{Pty, HANDLE};
 
 use std::i16;
+use std::io::Error;
 use std::mem;
 use std::os::windows::io::IntoRawHandle;
 use std::ptr;
@@ -155,11 +156,13 @@ pub fn new<'a>(
 
     // Create the appropriately sized thread attribute list.
     unsafe {
-        success =
+        let failure =
             InitializeProcThreadAttributeList(ptr::null_mut(), 1, 0, &mut size as PSIZE_T) > 0;
 
         // This call was expected to return false.
-        assert!(!success);
+        if (!failure) {
+            panic_shell_spawn();
+        }
     }
 
     let mut attr_list: Box<[BYTE]> = vec![0; size].into_boxed_slice();
@@ -183,7 +186,9 @@ pub fn new<'a>(
             &mut size as PSIZE_T,
         ) > 0;
 
-        assert!(success);
+        if (!success) {
+            panic_shell_spawn();
+        }
     }
 
     // Set thread attribute list's Pseudo Console to the specified ConPTY
@@ -198,7 +203,9 @@ pub fn new<'a>(
             ptr::null_mut(),
         ) > 0;
 
-        assert!(success);
+        if (!success) {
+            panic_shell_spawn();
+        }
     }
 
     // Get process commandline
@@ -231,7 +238,9 @@ pub fn new<'a>(
             &mut proc_info as *mut PROCESS_INFORMATION,
         ) > 0;
 
-        assert!(success);
+        if (!success) {
+            panic_shell_spawn();
+        }
     }
 
     // Store handle to console
@@ -253,6 +262,11 @@ pub fn new<'a>(
     })
 }
 
+// Panic with the last os error as message
+fn panic_shell_spawn() {
+    panic!("Unable to spawn shell: {}", Error::last_os_error());
+}
+
 impl OnResize for ConptyHandle {
     fn on_resize(&mut self, sizeinfo: &SizeInfo) {
         if let Some(coord) = coord_from_sizeinfo(sizeinfo) {