Strictly handle expected tag value types

Resolve several TODOs where tag values were cast from u32 without checks
by providing generic methods that error when the value fails casting to
the desired type. In addition to being more correct, this simplifies and
shortens decoding logic.

Remove `find_tag_u32` and `find_tag_u32_vec` as they are now redundant
and a GitHub code search found no usages outside copies of
`image-tiff`.

src/error.rs

@@ -38,8 +38,9 @@ pub enum TiffFormatError {
     ImageFileDirectoryNotFound,
     InconsistentSizesEncountered,
     InvalidTag,
+    InvalidTagValueType(Tag),
     RequiredTagNotFound(Tag),
-    UnknownPredictor(u32),
+    UnknownPredictor(u16),
     UnsignedIntegerExpected(Value),
     SignedIntegerExpected(Value),
     InflateError(InflateError),
@@ -57,6 +58,8 @@ impl fmt::Display for TiffFormatError {
             ImageFileDirectoryNotFound => write!(fmt, "Image file directory not found."),
             InconsistentSizesEncountered => write!(fmt, "Inconsistent sizes encountered."),
             InvalidTag => write!(fmt, "Image contains invalid tag."),
+            InvalidTagValueType(ref tag) =>
+                write!(fmt, "Tag `{:?}` did not have the expected value type.", tag),
             RequiredTagNotFound(ref tag) => write!(fmt, "Required tag `{:?}` not found.", tag),
             UnknownPredictor(ref predictor) => {
                 write!(fmt, "Unknown predictor “{}” encountered", predictor)
@@ -202,5 +205,17 @@ impl From<string::FromUtf8Error> for TiffError {
     }
 }
 
+impl From<TiffFormatError> for TiffError {
+    fn from(err: TiffFormatError) -> TiffError {
+        TiffError::FormatError(err)
+    }
+}
+
+impl From<TiffUnsupportedError> for TiffError {
+    fn from(err: TiffUnsupportedError) -> TiffError {
+        TiffError::UnsupportedError(err)
+    }
+}
+
 /// Result of an image decoding/encoding process
 pub type TiffResult<T> = Result<T, TiffError>;