[infer] Change the error message when the project root is not a prefix of the absolute path of the source files

Summary:
More verbose error in the case where the project root is not a prefix of the source file path

infer/src/backend/DB.ml

@@ -16,8 +16,6 @@ module L = Logging
 
 (** {2 Source Files} *)
 
-exception Path_not_prefix_root
-
 type source_file =
   | Absolute of string
   | Relative of string
@@ -51,7 +49,7 @@ let rel_source_file_from_abs_path root fname =
   if Utils.string_is_prefix root fname then
     let relative_fname = filename_to_relative root fname in
     Relative relative_fname
-  else raise Path_not_prefix_root
+  else failwith ("The project root " ^ root ^ " is not a prefix of " ^ fname)
 
 (** convert a path to a source file, turning it into an absolute path if necessary *)
 let abs_source_file_from_path fname =

infer/src/backend/DB.mli

@@ -60,8 +60,6 @@ end
 
 type source_file
 
-exception Path_not_prefix_root
-
 (** Maps from source_file *)
 module SourceFileMap : Map.S with type key = source_file
 

infer/src/clang/cLocation.ml

@@ -33,7 +33,9 @@ let source_file_from_path path =
   | Some root ->
       (try
          DB.rel_source_file_from_abs_path root path
-       with DB.Path_not_prefix_root ->
+       with Failure _ ->
+         Logging.out
+           "ERROR: %s should be a prefix of %s.@." root path;
          DB.source_file_from_string path)
   | None -> DB.source_file_from_string path