[SPARK-8902] Correctly print hostname in error

With "+" the strings are separate expressions, and format() is called on the last string before concatenation. (So substitution does not happen.) Without "+" the string literals are merged first by the parser, so format() is called on the complete string.

Should I make a JIRA for this?

Author: Daniel Darabos <darabos.daniel@gmail.com>

Closes #7288 from darabos/patch-2 and squashes the following commits:

be0d3b7 [Daniel Darabos] Correctly print hostname in error

ec2/spark_ec2.py

@@ -1153,8 +1153,8 @@ def ssh(host, opts, command):
                 # If this was an ssh failure, provide the user with hints.
                 if e.returncode == 255:
                     raise UsageError(
-                        "Failed to SSH to remote host {0}.\n" +
-                        "Please check that you have provided the correct --identity-file and " +
+                        "Failed to SSH to remote host {0}.\n"
+                        "Please check that you have provided the correct --identity-file and "
                         "--key-pair parameters and try again.".format(host))
                 else:
                     raise e