Clarify echo time errors. Closes #1219

Author: chrisgorgo

fmriprep/interfaces/fmap.py

@@ -517,9 +517,16 @@ def _delta_te(in_values, te1=None, te2=None):
             te1 = te1[1]
         if isinstance(te2, list):
             te2 = te2[1]
-
-    if te1 is None or te2 is None:
+    
+    # For convienience if both are missing we should give one error about them
+    if te1 is None and te2 is None:
+        raise RuntimeError(
+            'EchoTime1 and EchoTime2 metadata fields not found. Please consult the BIDS specification.')
+    if te1 is None:
+        raise RuntimeError(
+            'EchoTime1 metadata field not found. Please consult the BIDS specification.')
+    if te1 is None:
         raise RuntimeError(
-            'No echo time information found')
+            'EchoTime2 metadata field not found. Please consult the BIDS specification.')
 
     return abs(float(te2) - float(te1))