[TestBase.runCmd] Better error message when runCmd fails.

Summary:
Before:
    AssertionError: False is not True : Process is launched successfully

After:
    AssertionError: False is not True : Command 'run a.out' failed.
    >>> error: invalid target, create a target using the 'target create' command
    >>> Process could not be launched successfully

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: lldb-commits, vharron

Differential Revision: http://reviews.llvm.org/D9948

llvm-svn: 238363
diff --git a/lldb/test/settings/TestSettings.py b/lldb/test/settings/TestSettings.py
index 0f3c8b4..ff08215 100644
--- a/lldb/test/settings/TestSettings.py
+++ b/lldb/test/settings/TestSettings.py
@@ -227,7 +227,7 @@
         self.addTearDownHook(
             lambda: self.runCmd("settings clear target.env-vars"))
 
-        self.runCmd("run", RUN_SUCCEEDED)
+        self.runCmd("run", RUN_FAILED)
 
         # Read the output file produced by running the program.
         if lldb.remote_platform:
@@ -263,7 +263,7 @@
             os.environ.pop("MY_HOST_ENV_VAR2")
 
         self.addTearDownHook(unset_env_variables)
-        self.runCmd("run", RUN_SUCCEEDED)
+        self.runCmd("run", RUN_FAILED)
 
         # Read the output file produced by running the program.
         if lldb.remote_platform:
@@ -299,7 +299,7 @@
                     SETTING_MSG("target.output-path"),
                     substrs = ['target.output-path (file) = "stdout.txt"'])
 
-        self.runCmd("run", RUN_SUCCEEDED)
+        self.runCmd("run", RUN_FAILED)
 
         if lldb.remote_platform:
             self.runCmd('platform get-file "stderr.txt" "stderr.txt"')