display proper error message

- bug : https://bugs.launchpad.net/linaro-powerdebug/+bug/1014667

Signed-off-by: Sanjay Singh Rawat <sanjay.rawat@linaro.org>
diff --git a/display.c b/display.c
index 94ff2de..e9f4bf6 100644
--- a/display.c
+++ b/display.c
@@ -598,8 +598,10 @@
 {
 	size_t array_size = sizeof(windata) / sizeof(windata[0]);
 
-	if (win < 0 || win >= array_size)
+	if (win < 0 || win >= array_size) {
+		printf("error: invalid window");
 		return -1;
+	}
 
 	windata[win].ops = ops;
 
diff --git a/powerdebug.c b/powerdebug.c
index 098c5da..6cf3a1b 100644
--- a/powerdebug.c
+++ b/powerdebug.c
@@ -247,7 +247,7 @@
 	}
 
 	if (regulator_init()) {
-		printf("not enough memory to allocate regulators info\n");
+		printf("failed to initialize regulator\n");
 		options->regulators = false;
 	}
 
diff --git a/regulator.c b/regulator.c
index 0da924a..68c686f 100644
--- a/regulator.c
+++ b/regulator.c
@@ -218,8 +218,10 @@
 	struct regulator_info *reg;
 
 	reg = regulator_alloc();
-	if (!reg)
+	if (!reg) {
+		printf("error: unable to allocate memory for regulator\n");
 		return -1;
+	}
 	t->private = reg;
 
         /* we skip the root node but we set it expanded for its children */
diff --git a/tree.c b/tree.c
index d331c60..686e846 100644
--- a/tree.c
+++ b/tree.c
@@ -121,8 +121,10 @@
 	int ret = 0;
 
 	dir = opendir(tree->path);
-	if (!dir)
+	if (!dir) {
+		printf("error: unable to open directory %s\n", tree->path);
 		return -1;
+	}
 
 	while (!readdir_r(dir, &dirent, &direntp)) {