Add better messages on environment variable error

Otherwise it doesn't indicate the name of the environment variable in
the result output:

    Error: Error(EnvVar(NotPresent), State { next_error: None, backtrace: InternalBacktrace { backtrace: None } })

1 files changed, 2 insertions, 1 deletions

diff --git a/license-generator/src/database.rs b/license-generator/src/database.rs
index 94e925f..505a788 100644
--- a/license-generator/src/database.rs
+++ b/license-generator/src/database.rs
@@ -5,7 +5,8 @@ use mysql;
 use errors::*;
 
 pub fn get_database_connection() -> Result<mysql::PooledConn> {
-    let connection_url = env::var("DATABASE_URL")?;
+    let connection_url = env::var("DATABASE_URL")
+        .chain_err(|| "DATABASE_URL environment variable not found")?;
     let pool = mysql::Pool::new_manual(10, 50, connection_url)?;
     let cx = pool.get_conn()?;