provider/aws: Provide a better message if no AWS creds are found

builtin/providers/aws/config.go

@@ -120,7 +120,13 @@ func (c *Config) Client() (interface{}, error) {
 		// error, and we can present it nicely to the user
 		_, err = creds.Get()
 		if err != nil {
-			errs = append(errs, fmt.Errorf("Error loading credentials for AWS Provider: %s", err))
+			if awsErr, ok := err.(awserr.Error); ok && awsErr.Code() == "NoCredentialProviders" {
+				errs = append(errs, fmt.Errorf(`No valid credential sources found for AWS Provider. 
+  Please see https://terraform.io/docs/providers/aws/index.html for more information on 
+  providing credentials for the AWS Provider`))
+			} else {
+				errs = append(errs, fmt.Errorf("Error loading credentials for AWS Provider: %s", err))
+			}
 			return nil, &multierror.Error{Errors: errs}
 		}
 		awsConfig := &aws.Config{

website/source/docs/providers/aws/index.html.markdown

@@ -30,6 +30,23 @@ resource "aws_instance" "web" {
 }
 ```
 
+## Authentication 
+
+The AWS provider offers flexible means of providing credentials for
+authentication. Included is support including hard coded credentials, 
+environment variables, and shared credential files, in that order of precedence. 
+
+Terraform will first attempt to use an `access_key` and `secret_key` provided in 
+the `provider` block (shown in the example above). If those are omitted, it will 
+attempt to discover those values by referencing the `AWS_ACCESS_KEY_ID` and 
+`AWS_SECRET_ACCESS_KEY` environment variables. Lastly, if those are not found 
+it will look for credentials in the default location for a credentials file, or 
+the file path specified in the `shared_credentials_file` attribute of the 
+`provider` block.
+
+See the argument reference below for information on which attributes to specify
+to use a corresponding credential provider.  
+
 ## Argument Reference
 
 The following arguments are supported in the `provider` block: