Improve shebang execution error message (#184)

I was reusing TmpdirIoError for a few cases, but one of them
usually has more to do with the contents of the shebang line than
an actual io error involving the tmpdir. Pull it out into its own
RunError variant and improve the message.

src/lib.rs

@@ -418,8 +418,12 @@ impl<'a> Recipe<'a> {
         } else {
           return Err(error_from_signal(self.name, None, exit_status))
         },
-        Err(io_error) => return Err(RunError::TmpdirIoError{
+        Err(io_error) => return Err(RunError::Shebang {
-          recipe: self.name, io_error: io_error})
+          recipe:   self.name,
+          command:  shebang_command.to_string(),
+          argument: shebang_argument.map(String::from),
+          io_error: io_error
+        })
       };
     } else {
       let mut lines = self.lines.iter().peekable();
@@ -1330,19 +1334,20 @@ impl<'a> Display for Justfile<'a> {
 #[derive(Debug)]
 enum RunError<'a> {
   ArgumentCountMismatch{recipe: &'a str, found: usize, min: usize, max: usize},
-  Code{recipe: &'a str, line_number: Option<usize>, code: i32},
-  InternalError{message: String},
-  IoError{recipe: &'a str, io_error: io::Error},
-  Signal{recipe: &'a str, line_number: Option<usize>, signal: i32},
-  TmpdirIoError{recipe: &'a str, io_error: io::Error},
-  UnknownFailure{recipe: &'a str, line_number: Option<usize>},
-  UnknownRecipes{recipes: Vec<&'a str>, suggestion: Option<&'a str>},
-  UnknownOverrides{overrides: Vec<&'a str>},
   BacktickCode{token: Token<'a>, code: i32},
   BacktickIoError{token: Token<'a>, io_error: io::Error},
   BacktickSignal{token: Token<'a>, signal: i32},
-  BacktickUtf8Error{token: Token<'a>, utf8_error: std::str::Utf8Error},
   BacktickUnknownFailure{token: Token<'a>},
+  BacktickUtf8Error{token: Token<'a>, utf8_error: std::str::Utf8Error},
+  Code{recipe: &'a str, line_number: Option<usize>, code: i32},
+  InternalError{message: String},
+  IoError{recipe: &'a str, io_error: io::Error},
+  Shebang{recipe: &'a str, command: String, argument: Option<String>, io_error: io::Error},
+  Signal{recipe: &'a str, line_number: Option<usize>, signal: i32},
+  TmpdirIoError{recipe: &'a str, io_error: io::Error},
+  UnknownFailure{recipe: &'a str, line_number: Option<usize>},
+  UnknownOverrides{overrides: Vec<&'a str>},
+  UnknownRecipes{recipes: Vec<&'a str>, suggestion: Option<&'a str>},
 }
 
 impl<'a> Display for RunError<'a> {
@@ -1388,6 +1393,15 @@ impl<'a> Display for RunError<'a> {
           write!(f, "Recipe `{}` failed with exit code {}", recipe, code)?;
         }
       },
+      Shebang{recipe, ref command, ref argument, ref io_error} => {
+        if let Some(ref argument) = *argument {
+          write!(f, "Recipe `{}` with shebang `#!{} {}` execution error: {}",
+                    recipe, command, argument, io_error)?;
+        } else {
+          write!(f, "Recipe `{}` with shebang `#!{}` execution error: {}",
+                    recipe, command, io_error)?;
+        }
+      }
       Signal{recipe, line_number, signal} => {
         if let Some(n) = line_number {
           write!(f, "Recipe `{}` was terminated on line {} by signal {}", recipe, n, signal)?;