Improve error behavior (#15)

* Executing a blocking oneshot command should not easily throw Exceptions, but return an error result instead.

* Javadocs

core/src/main/java/eu/darken/rxshell/cmd/Cmd.java

@@ -5,6 +5,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 import java.util.UUID;
 
@@ -300,9 +301,12 @@ public Single<Result> submit(RxCmdShell shell) {
 
         /**
          * Convenience method for {@link #submit(RxCmdShell)} using {@link Single#blockingGet()}
+         * <br>If a shell can't be opened {@link ExitCode#EXCEPTION} will be returned and an error message.
          */
         public Result execute(RxCmdShell shell) {
-            return submit(shell).blockingGet();
+            return submit(shell)
+                    .onErrorReturn(err -> new Result(build(), ExitCode.EXCEPTION, null, Collections.singletonList(err.toString())))
+                    .blockingGet();
         }
     }
 

core/src/test/java/eu/darken/rxshell/cmd/CmdBuilderTest.java

@@ -150,12 +150,15 @@ public void testExecute_oneshot() {
         verify(session).close();
     }
 
-    @Test(expected = RuntimeException.class)
+    @Test
     public void testExecute_oneshot_exception() throws IOException {
         RxCmdShell shell = mock(RxCmdShell.class);
-        when(shell.open()).thenReturn(Single.error(new IOException()));
+        Exception ex = new IOException("Error message");
+        when(shell.open()).thenReturn(Single.error(ex));
         when(shell.isAlive()).thenReturn(Single.just(false));
 
-        Cmd.builder("").execute(shell);
+        final Cmd.Result result = Cmd.builder("").execute(shell);
+        assertThat(result.getExitCode(), is(Cmd.ExitCode.EXCEPTION));
+        assertThat(result.getErrors(), contains(ex.toString()));
     }
 }