Start implementing error explanations

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diff --git a/src/errors/InvalidPredicate.txt b/src/errors/InvalidPredicate.txt
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+Explanation: Every ❰if❱ expression begins with a predicate which must have type
+❰Bool❱
+
+For example, these are valid ❰if❱ expressions:
+
+
+    ┌──────────────────────────────┐
+    │ if True then "Yes" else "No" │
+    └──────────────────────────────┘
+         ⇧
+         Predicate
+
+
+    ┌─────────────────────────────────────────┐
+    │ λ(x : Bool) → if x then False else True │
+    └─────────────────────────────────────────┘
+                       ⇧
+                       Predicate
+
+
+... but these are $_NOT valid ❰if❱ expressions:
+
+
+    ┌───────────────────────────┐
+    │ if 0 then "Yes" else "No" │  ❰0❱ does not have type ❰Bool❱
+    └───────────────────────────┘
+
+
+    ┌────────────────────────────┐
+    │ if "" then False else True │  ❰""❱ does not have type ❰Bool❱
+    └────────────────────────────┘
+
+
+Your ❰if❱ expression begins with the following predicate:
+
+↳ $txt0
+
+... that has type:
+
+↳ $txt1
+
+... but the predicate must instead have type ❰Bool❱
+
+Some common reasons why you might get this error:
+
+● You might be used to other programming languages that accept predicates other
+  than ❰Bool❱
+
+  For example, some languages permit ❰0❱ or ❰""❱ as valid predicates and treat
+  them as equivalent to ❰False❱.  However, the Dhall language does not permit
+  this