Remove dependency on crate::Error from arg_defs

This change removes the need to import crate::Error from the arg_defs
module. By dropping this dependency we make the file more independent of
the rest of the crate, which subsequently will allow us to merely
include! it in another file in order to get the argument related type
definitions without compilation errors due to missing symbols from the
rest of the crate.

src/arg_defs.rs

@@ -19,7 +19,6 @@
 
 use crate::args;
 use crate::commands;
-use crate::error::Error;
 
 /// Provides access to a Nitrokey device
 #[derive(structopt::StructOpt)]
@@ -136,13 +135,13 @@ pub enum ConfigOption<T> {
 }
 
 impl<T> ConfigOption<T> {
-  pub fn try_from(disable: bool, value: Option<T>, name: &'static str) -> Result<Self, Error> {
+  pub fn try_from(disable: bool, value: Option<T>, name: &'static str) -> Result<Self, String> {
     if disable {
       if value.is_some() {
-        Err(Error::Error(format!(
+        Err(format!(
           "--{name} and --no-{name} are mutually exclusive",
           name = name
-        )))
+        ))
       } else {
         Ok(ConfigOption::Disable)
       }

src/error.rs

@@ -64,6 +64,12 @@ impl From<&str> for Error {
   }
 }
 
+impl From<String> for Error {
+  fn from(s: String) -> Error {
+    Error::Error(s)
+  }
+}
+
 impl From<clap::Error> for Error {
   fn from(e: clap::Error) -> Error {
     Error::ClapError(e)