Add LoaderError and WriterError exceptions

When throwing these errors in library code, FileIO will
emit an info message and then try the next registered loader / writer.

These are useful for marking specific failed assertions.  If the file
does not conform to what the loader expects, it can bail out and allow
the next reader to attempt to load the file.  At the same time,
legitimate programming errors will halt execution with an error message.

Author: rsrock

.travis.yml

@@ -3,7 +3,8 @@ os:
   - linux
   - osx
 julia:
-  - release
+  - 0.3
+  - 0.4
   - nightly
 notifications:
   email: false

src/FileIO.jl

@@ -36,11 +36,35 @@ export DataFormat,
        stream,
        unknown
 
+import Base.showerror
+
 include("query.jl")
 include("loadsave.jl")
 include("registry.jl")
 
+@doc """
+`LoaderError` should be thrown when loader library code fails, and other libraries should
+be given the chance to recover from the error.  Reports the library name and an error message:
+LoaderError("ImageMagick", "Foo not available")
+""" ->
+type LoaderError <: Exception
+    library::UTF8String
+    msg::UTF8String
+end
+Base.showerror(io::IO, e::LoaderError) = println(io, e.library, " load error: ",
+                                                 msg, "\n  Will try next loader.")
 
+@doc """
+`WriterError` should be thrown when writer library code fails, and other libraries should
+be given the chance to recover from the error.  Reports the library name and an error message:
+WriterError("ImageMagick", "Foo not available")
+""" ->
+type WriterError <: Exception
+    library::UTF8String
+    msg::UTF8String
+end
+Base.showerror(io::IO, e::WriterError) = println(io, e.library, " writer error: ",
+                                                 msg, "\n  Will try next writer.")
 
 @doc """
 - `load(filename)` loads the contents of a formatted file, trying to infer
@@ -59,10 +83,14 @@ function load(s::@compat(Union{AbstractString,IO}), args...; options...)
             Library = check_loader(library)
             return Library.load(q, args...; options...)
         catch e
-            last_exception = e
+            if isa(e, LoaderError)
+                info(e.library, " failed. ", e.msg)
+                info("  Will try next loader, if available")
+            else
+                rethrow(e)
+            end
         end
     end
-    rethrow(last_exception)
 end
 
 @doc """
@@ -80,10 +108,14 @@ function save(s::@compat(Union{AbstractString,IO}), data...; options...)
             Library = check_saver(library)
             return Library.save(q, data...; options...)
         catch e
-            last_exception = e #TODO, better and standardized exception propagation system
+            if isa(e, WriterError)
+                info(e.library, " failed. ", e.msg)
+                info("  Will try next writer, if available")
+            else
+                rethrow(e)
+            end
         end
     end
-    rethrow(last_exception)
 end
 
 # Forced format
@@ -108,10 +140,14 @@ function Base.writemime(io::IO, mime::MIME, x)
             check_mime(pkg)
             return writemime(Stream(DataFormat{pkg}, io), mime, x)
         catch e
-            last_exception = e
+            if isa(e, WriterError)
+                info(e.library, " failed. ", e.msg)
+                info("  Will try next writer, if available")
+            else
+                rethrow(e)
+            end
         end
     end
-    rethrow(last_exception)
 end
 
 # Fallbacks